[JAX] Added prepare phase for the FusedAttnForwardFFI

[phu0ngng]

Description

In this PR, the FusedAttnForwardFFI is registered to XLA via 2 phases:

1. prepare where the te_cudnn_handle_init_ffi is called.
2. execute phase where the actual fused attention function is called. In the past, the default was to have only the execute phase. This helps to avoid memory allocation for the handle in the cudaGraph capturing area, thus resolving the issue we have in the distributed run with cudaGraph.

In general, all the custom calls that involve cuDNN kernels should be registered with these two phases in the future. Since the prepare function needs to have the same function argument list with the execute function ones, variadic args are used to make this te_cudnn_handle_init_ffi general so that it can be called in any custom calls.

Type of change

• [ ] Documentation change (change only to the documentation, either a fix or a new content)
• [x] 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 refractor

Checklist:

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

[phu0ngng]

/te-ci jax L1

[phu0ngng]

/te-ci jax L1