webgpu: Fix the maxpool error (#4854)
webgpu: Fix the maxpool error (#4854)

BUG

Should use '-1.0 / 1e-20' instead of '0.0' as the initial value for
maxPool. Otherwise, it will get incorrect result if the value in input
is negative.

Meanwhile, this patch also refactors the implementation of maxpool and
uses flatten dispatching layout.
tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs,tensorflow/tfjs