Proposal: Add an optional mask argument to every function involving a reduction

[carlosgmartin]

Proposal: Add an optional mask: array | None argument to every function involving a reduction, indicating which elements of the input participate in the reduction. The default would be None, indicating that all elements participate.

This includes the following functions:

• argmax
• argmin
• cumulative_sum
• max
• mean
• min
• prod
• std
• sum
• var
• all
• any

Context:

• Add where argument to argmax, argmin, ptp, cumsum, cumprod jax-ml/jax#20177
• ENH: Add where argument to reduction functions that are missing it numpy/numpy#26336
• Add mask argument to lax.argmax jax-ml/jax#25623