Add rewrites to convert reshape to expand_dims or static_slice when possible

[copybara-service]

Add rewrites to convert reshape to expand_dims or static_slice when possible

Generated by AI first, and then I partially rewrote the rewrite to be simpler and smaller. Here is what it had to say:

✦ I have added a rewrite rule to fusion.cc that converts reshape operations into either expand_dims or slice (squeeze) when possible.

Summary of Changes

1. Refactored subgraph/copy.cc: Extracted the core logic for defining static_reshape and static_expand_dims into internal C++ functions (define_static_reshape and define_static_expand_dims) and exported them via subgraph/copy.h.
2. Updated subgraph/static_slice.cc: Similarly refactored static_slice logic into a reusable define_static_slice function and created a new internal header subgraph/static_slice.h for it.
3. Modified subgraph/fusion.cc:
   • Implemented rewrite_reshape, which identifies if a static_reshape only adds or removes dimensions of size 1.
   • If it adds dimensions, it rewrites it to a static_expand_dims node.
   • If it removes dimensions, it rewrites it to a static_slice node with the slice_dims flag enabled (effectively a squeeze).
   • Integrated this new rewrite into the main fusion loop.
4. Enhanced Testing Infrastructure:
   • Added IsStaticReshape, IsStaticExpandDims, and IsStaticSlice matchers to subgraph/test/matchers.h.
   • Created subgraph/test/fusion_reshape.cc with unit tests for these new rewrites, ensuring that reshape is correctly converted when possible and remains unchanged when it performs a more complex transformation.
5. Build System Updates: Updated subgraph/BUILD and subgraph/test/BUILD to include the new files and test cases.

All tests passed successfully, confirming the correctness of the rewrites.