Add SFT Recipe

[jacklanchantin]

What does this PR do? Please describe:
A summary of the change or the issue that is fixed.

• Adds SFT recipe for masked input training.
• Adapts old instruction_finetune recipe for to fit within v0.5 train recipe format.

Tested 4 configs using named assets:

dataset=alpaca, model=llama_3_2_1b
dataset=alpaca, model=llama_3_2_1b_instruct, chat_mode=true
dataset=gsm8k, model=llama_3_2_1b
dataset=gsm8k, model=llama_3_2_1b_instruct, chat_mode=true

Run using:

python -m sft /output/dir/ --config-file sft/configs/llama3_2_1b_gsm8k.yaml

Does your PR introduce any breaking changes? If yes, please list them:
List of all backwards-incompatible changes.

Check list:

• Was the content of this PR discussed and approved via a GitHub issue? (no need for typos or documentation improvements)
• Did you read the contributor guideline?
• Did you make sure that your PR does only one thing instead of bundling different changes together?
• Did you make sure to update the documentation with your changes? (if necessary)
• Did you write any new necessary tests?
• Did you verify new and existing tests pass locally with your changes?
• Did you update the CHANGELOG? (no need for typos, documentation, or minor internal changes)