Fix test of FSDP2 by correcting init logic and applying autocast

[ntenenz]

Description

This fixes multiple issues in the FSDP2 test, namely

1. Previously fp8 init was performed when args.fp8_init == False. I have updated the logic to match what I presume was intended by leveraging the nullcontext context manager.
2. te.fp8_autocast was previously not called; the recipe was created but was unused. The autocast context manager now wraps the model's computation.

Does not necessarily fix #1705, but it does enable the test which should determine if the issue is still present.

Type of change

• Documentation change (change only to the documentation, either a fix or a new content)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Infra/Build change
• Code refactoring

Changes

Please list the changes introduced in this PR:

• Enable / disable fp8_model_init according to the test's flag
• Apply fp8_autocast

Checklist:

• I have read and followed the contributing guidelines
• The functionality is complete
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[timmoon10]

/te-ci pytorch L1

[timmoon10]

LGTM, pending CI

[timmoon10]

/te-ci pytorch L1