Fix prod dtype promotion when reducing a size-1 axis

[eyupcanakman]

Proposed changes

prod computes its promoted output type but discards it on the no-op branch, so the result dtype depends on the length of the reduced axis. Reducing a size-1 axis returns the input dtype while a size-2 axis promotes:

mx.prod(mx.array([2], dtype=mx.int8), axis=0).dtype     # int8   (before)
mx.prod(mx.array([2, 2], dtype=mx.int8), axis=0).dtype  # int32

sum casts on the same branch (is_noop ? astype(a, out_type, s) : ...), but prod returned bare a. The fix applies the same astype, so prod matches sum and the other promoting reductions. The promotion is there to keep the product from overflowing the narrow input type, so the size-1 case was also silently keeping a narrow accumulator.

Added a test that the output dtype does not depend on the reduced axis length.

Checklist

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• I have read the CONTRIBUTING document
• I have run pre-commit run --all-files to format my code / installed pre-commit prior to committing changes
• I have added tests that prove my fix is effective or that my feature works
• I have updated the necessary documentation (if needed)

[angeloskath]

👍