Fix gradient for min and max

[tafsiri]

Closes #4135
Adds a regression test to guard against #4130 (which seems to be fixed at HEAD at the time of that issue being reported).

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[lina128]

Thank you Yannick! Can you provide some context here, why we don't need transpose any more?

Reviewable status: complete! 1 of 1 approvals obtained (waiting on @annxingyuan and @lina128)

[tafsiri]

Good question. There were a few things I noticed.

• The grad function would itself try and transpose the output if permutedAxes was not null. However the gradForMinAndMax helper function would also do this as part of the helper function, effectively nullifying each other (though the outer transpose was called without permutedAxes which just seemed odd).
• The min and max grad functions seemed to be written against the eariler definition of the kernel in the forward func. For example looking at how the code was at 1.7.4, you can see that that op would sometimes to an internal transpose before passing the input to the backend implementation, however the original input is what gets passed to the gradient. So I believe the gradient was trying to match the shape of what gets passed to backend.max internally. However in new modular kernels there is no op level transformation and thus no transpose to account for (any internal transposition done within the forward kernel are completely hidden). So the extra 'transposition handling' code becomes unnecessary.

[lina128]

Ah, makes sense!