[XLA] Initialize arrays using cudaMemset when possible.

[XLA] Initialize arrays using cudaMemset when possible.

Previously we were using our own hand-rolled initializer thunk.  This
worked OK for reduces, because the amount of data we were initializing
is usually small.  But for e.g. select-and-scatter, it's quite slow.

This patch lets us use cudaMemset instead.

PiperOrigin-RevId: 189904720