Advanced NLP
Lecture-1: Introduction
source: https://www.youtube.com/watch?v=dir7e3uozQU
Housekeeping stuff
- Course website: https://people.cs.umass.edu/~miyyer/cs685
- Schedule: Schedule - CS 685, Spring 2024, UMass Amherst
- Submit questions/concerns/feedback to https://forms.gle/wtSgjAQ3aa9z29ux5
- LM Conference: COLM 2024: Call For Papers (colmweb.org)
Timestamp
00:00:37 Course Logistics
00:17:30 Natural Language Processing
00:26:00 Supervised Learning
00:26:45 Self-Supervised Learning
00:30:05 Transfer Learning
00:34:55 In-Context Learning
00:37:00 LLM shortcomings
01:04:55 LLM usage
01:07:36 List of topics of the course
01:09:20 Final projects
Lecture notes
Supervised Learning
- Given a collection of labeled examples (where each example is a text X paired with a label Y), learn a mapping from X to Y.
- Example: given a collection of 20K movie reviews, train a model to map review text to review score (sentiment analysis)
Self-Supervised Learning
- Given a collection of just text, without extra labels, create labels out of the text and use them to pre-train a model with some general understanding of human language.
- Language modeling: given the beginning of a sentence or document, predict the next word
- Masked language modeling: given an entire document with some words or spans masked out, predict the missing words
Supervised learning relies on labeled datasets to learn a direct mapping from inputs to outputs. Example: Image classification
Self-supervised learning leverages the structure within the data to create a learning signal, enabling the model to learn representations that can be used for various tasks without the need for explicit labels. Example: LLM
Transfer learning
First, pre-train a large self-supervised model and then fine-tune it on a small labeled dataset using supervised learning.
Example: pre-train a large language model on hundreds of billions of words, and then fine-tune it on 20K reviews to specialize it for sentiment analysis.
In-context learning
- First, pre-train a large self-supervised model and then prompt it in natural language to solve a particular task without further training.
- Example: pre-train a large language model on hundreds of billions of words, and then feed in "what is the sentiment of this sentence: <insert sentence>"
LLM Usage
These are the most used cases for which LLM has been used in descending order[1].
Cluster 1: Discussing software errors and solutions
Cluster 2: Inquiries about Al tools, software design, and programming
Cluster 3: Geography, travel, and global cultural inquiries
Cluster 4: Requests for summarizing and elaborating texts
Cluster 5: Creating and improving business strategies and products
Cluster 6: Requests for Python coding assistance and examples
Cluster 7: Requests for text translation, rewriting, and summarization
Cluster 8: Role-playing various characters in conversations
Cluster 9: Requests for explicit and erotic storytelling
Cluster 10: Answering questions based on passages
Cluster 11: Discussing and describing various characters
Cluster 12: Generating brief sentences for various job roles
Cluster 13: Role-playing and capabilities of Al chatbots
Cluster 14: Requesting introductions for various chemical companies
Cluster 15: Explicit sexual fantasies and role-playing scenarios
Cluster 16: Generating and interpreting SQL queries from data
Cluster 17: Discussing toxic behavior across different identities
Cluster 18: Requests for Python coding examples and outputs
Cluster 19: Determining factual consistency in document summaries
Cluster 20: Inquiries about specific plant growth conditions
List of topics of the course
- Background: language models and neural networks
- Models: RNNs > Transformers
- ELMo > BERT > GPT3 > ChatGPT > today's LLMs
- Tasks: text generation (e.g., translation, summarization), classification, retrieval, etc.
- Data: annotation, evaluation, artifacts
- Methods: pretraining, finetuning, preference tuning, prompting
Lecture-2: N-gram LM
source: UMass CS685 S24 (Advanced NLP) #2: N-gram language models (youtube.com)
Timestamp
00:02:30 Background
00:08:10 Probabilistic Language Model
00:16:50 Markov Assumption
00:31:23 Comparing Unigram, Bigram,...Ngram models
00:47:50 Infini-gram: a state-of-the-art n-gram model on I.4T tokens
00:53:50 LM Evaluation
00:57:00 Perplexity
01:05:26 Notion of Sparsity
01:08:50 Smoothing
Lecture notes
Ngram models
- We can extend to trigrams, 4-grams, 5-grams
- In general, this is an insufficient model of language because language has long-distance dependencies:
For example, "The computer I had just put into the machine room on the fifth floor crashed."
A word type is the abstract concept of a word, while a word token is a specific instance of that word in use. The sentence "A rose is a rose is a rose" contains three word types ("a," "rose," and "is") but nine-word tokens, as each word is counted every time it appears.
Evaluation: How good is our model?
- Does our language model prefer good sentences to bad ones?
- Assign higher probability to "real" or "frequently observed" sentences than "ungrammatical" or "rarely observed" sentences.
- We train the parameters of our model on a training set.
- We test the model's performance on data we haven't seen.
- A test set is an unseen dataset different from our training set and unused.
- An evaluation metric tells us how well our model does on the test set.
- The goal isn't to pound out fake sentences!
- Generated sentences get "better" as we increase the model order.
- More precisely, using maximum likelihood estimators, higher order is always better likelihood on the training set, but not the test set.
Perplexity
- A higher perplexity score means poor performance by the model. The model is unable to find any relevancy based on the prefix/context provided, so it gives a uniform score to possible following words.
- A model that has certainty gets a lower score.
- Generally, human text has a higher perplexity score than machine-generated text.
Lower perplexity = Better model
Lecture 3: Neural language models (forward propagation)
source: https://www.youtube.com/live/qESAY4AE2IQ?si=-PP8gOS4ZX-8oiNh
Timestamp
00:03:14 Ngram model shortcomings
00:15:35 One hot vector in the Ngram model
00:20:10 Neural Language Model Introduction
00:26:53 Composing Word embeddings
00:43:33 Probability of word distribution by Softmax function
00:47:01 Neural network output layer
00:49:31 Composite functions for the prefix
00:52:20 Fixed window language model
00:57:57 Concatenation-based Neural model vs. Ngram model
01:01:12 RNN (Recurrent Neural Networks) Language model
Lecture notes
Issues with the Ngram model
- Sparsity
- Space
- No context of the words
Concatenation LM
Improvements over n-gram LM:
- No sparsity problem
- Model size is O(n) not O(exp(n))
Remaining problems: - Fixed window is too small
- Enlarging window enlarges W
- Window can never be large enough!
- Each c, uses different rows of W. We don't share weights across the window.
RNN
RNN Advantages:
- Can process any length input
- Model size doesn't increase for longer input
- Computation for step t can (in theory) use information from many steps back
- Weights are shared across timesteps -> representations are shared
RNN Disadvantages:
- Recurrent computation is slow
- In practice, difficult to access information from many steps back
Lecture-4: Neural Language Model (Backpropagation)
source: https://www.youtube.com/watch?v=MEVlSXJUBzI
Timestamp
00:02:05 Recap on Forward propagation
00:10:40 Train a Neural LM
00:20:25 Minimizing Loss
00:30:30 Hyperparameter
