Add implicit downcast in TMA descriptor store

[codex-maintainers]

Description

This fixes a missing implicit downcast when storing blocks through TMA descriptors. Previously, attempting to widen the result of a descriptor load (e.g. from float16 to float32) and then store it back via the descriptor would result in an MLIR verification error because the block types no longer matched:

# ptr.element_ty is tl.float16
desc = tl._experimental_make_tensor_descriptor(ptr, shape=.., strides=..., block_shape=...)
value = desc.load([off_x, off_y]).to(tl.float32)
# 'tt.experimental_descriptor_store' op tensor desciptor block and tensor types must match
desc.store([off_x, off_y], value)

The pointer/tl.store path already cast values to the target element type; descriptor stores should behave the same.

Changes

• Updated descriptor_store in python/triton/language/semantic.py to cast the incoming tensor to the descriptor's element type before emitting the create_descriptor_store IR node.
• Added a regression test test_tensor_descriptor_store_downcast to python/test/unit/cuda/test_experimental_tma.py which widens a float16/bfloat16 block to float32 and stores it back via the descriptor.
• Ran pre-commit hooks to keep formatting consistent.
  A quick check under TRITON_INTERPRET=1 shows the new downcast path works:

True  # torch.equal(a, out) when storing a widened float16 block
True  # bfloat16 as well

Checklist

• 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.
• I have added tests under python/test.
• I have not added any lit tests.

---

This fix aligns descriptor stores with pointer store semantics and avoids an IR verifier failure when the stored block's element type is wider than the descriptor’s element type.

[Jokeren]

@Mogball @gopoto Should the descriptor store be fully explicit or we can allow implicit cast?

[ThomasRaoux]

@Mogball @gopoto Should the descriptor store be fully explicit or we can allow implicit cast?

I believe we want to allow implicit cast, this matches the behavior of tl.store