Fixes for MM Masking and Collation

[pbontrager]

Context

What is the purpose of this PR? Is it to

• add a new feature
• fix a bug
• update tests and/or documentation
• other (please add here)

Several bug fixes here:

• left padding needs to pad encoder_mask on the left
• encoder_mask transform max size is based on max_num_tiles not encoder_max_seq_len
• collate needs to support optional pad_max_images for when a kv_cache with a fixed size is set.

Changelog

transforms.py and _collate.py along with tests updated

Test plan

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UX

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torchtune/torchtune/modules/vision_transformer.py

Line 285 in 6a7951f

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Example in our docs: https://pytorch.org/torchtune/main/tutorials/qat_finetune.html#applying-qat-to-llama3-models

• I did not change any public API;
• I have added an example to docs or docstrings;

[pytorch-bot]

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[joecummings]

PRETTTTYyyyyy

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.77%. Comparing base (726abb0) to head (d9acfd8).
Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1601      +/-   ##
==========================================
+ Coverage   70.72%   70.77%   +0.05%     
==========================================
  Files         288      288              
  Lines       14213    14238      +25     
==========================================
+ Hits        10052    10077      +25     
  Misses       4161     4161              

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