[TUTORIAL] non-causal mode in fused-attention backward

[arthurfeeney]

Description

This makes some simple additions to add support for non-causal attention in 06-fused-attention.py. The forward pass already uses the causal parameter, so this just changes the backward pass.

I think having the non-causal mode supported makes the code a little easier to understand. I.e., I did not find it obvious why the backward pass always masked the diagonal blocks until I realized it was just always computing the causal version.

New contributor declaration

• I am not making a trivial change, such as fixing a typo in a comment.

• I have written a PR description following these
  rules.

• I have run pre-commit run --from-ref origin/main --to-ref HEAD.

• Select one of the following.

  • I have added tests.
    • /test for lit tests
    • /unittest for C++ tests
    • /python/test for end-to-end tests
  • This PR does not need a test because The script includes a simple test, so I set it to run the non-causal mode as well.

• Select one of the following.

  • I have not added any lit tests.
  • The lit tests I have added follow these best practices,
    including the "tests should be minimal" section. (Usually running Python code
    and using the instructions it generates is not minimal.)