Amax for FP8 Convolutions in XLA
CC @reedwm.
Extends the functionality of scaled convolutions operating on `F8E4M3FN` and `F8E5M2` data types to optionally return the scalar maximum of the absolute (Amax) of the result before quantization,

(X, W, x_scale, w_scale, y_scale) -> (Y, y_amax).
