mul: remove opmath cast sequence

[sshonTT]

Remove the explicit opmath-driven cast chain (bf16→f32→bf16, etc.) from mul. The op now executes in the dtype chosen by standard dtype promotion, without inserting unconditional upcast/downcast steps. But leave its functionality for future usage.

[sshonTT]

Fixes issue #9662

[sshonTT]

@ysiraichi Hi, could you take a look when you have a chance?

[ysiraichi]

Could you add a test to verify whether that does what we expect?

[sshonTT]

Could you add a test to verify whether that does what we expect?

I could add a test for the mul op, but as you know, this patch affects the intermediate type casting, so it’s tricky to capture the cast → op → cast-back sequence. Do you have any ideas on how to validate this?

[ysiraichi]

You can use expecttest!
See test_ops_error_message.py for reference.

[sshonTT]

@ysiraichi ,I was going to try the expecttest module you suggested, but I found that test_operations_hlo.py already provides a more effective way to check the behavior I wanted. So I added the test there instead.

[ysiraichi]

Thank you for the PR.