[Train] Training Pipeline

[horheynm]

Order of reviews:
#1206
#1207
#1209
#1212
#1214 <-- Here

SUMMARY:

• Refactor Training pipeline
• Remove initialize, finalize from the session functions
• Add training information on entrypoints/readme.md on the different types of training that can be carried out on llm-compressor
• Decouple training from text_generation.py::main. The new logic loves in llmcompressor/entrypoints/train.py that takes the flow of pre-process, carry out training logic and then post-process
• Delete outdated info on transformers/finetune/readme.md
• Update session_mixin.py to use session().initialize or session().finalize.
• Deprecate train.py in text_generation.py, raising deprecation message if used.
• Update tests to use llmcompressor's train, not llmcompressor.transformers' train

TEST PLAN:

• Pass tests

[github-actions]

👋 Hi! Thank you for contributing to llm-compressor. Please add the ready label when the PR is ready for review.

Note: This is required to complete the testing suite, please only add the label once the PR is code complete and local testing has been performed.

[rahul-tuli]

LGTM pending tests!

[dsikka]

Please write a more descriptive PR description, summarizing changes and test steps.
The current description isn't very helpful.

[dsikka]

LGTM given one comment.
We've validated with our current examples:

1. https://github.com/vllm-project/llm-compressor/tree/main/examples/trl_mixin
2. https://github.com/vllm-project/llm-compressor/blob/main/examples/quantization_2of4_sparse_w4a16/llama7b_sparse_w4a16.py

run to completion, outputs make sense?