Rename GlobalAveragePooling to GlobalAvgPool

[jekbradbury]

For consistency with the other pooling APIs. Also updates to use squeezingAxes (should be NFC).

[Shashi456]

@jekbradbury why is squeezing axes a better approach than the earlier one?

[rxwei]

@jekbradbury why is squeezing axes a better approach than the earlier one?

It calls fewer TensorFlow ops. mean(squeezingAxis:) is equivalent to tf.reduce_mean(..., keep_dims=false). Though eventually we'd expect the compiler to optimize all of this away and make the performance be equivalent, the reality today is that the fewer op dispatches the better.

[Shashi456]

@rxwei this wouldn't solve the non-differentiable error that pops up in #76 right?

[rxwei]

The non-differentiable error there is because max(squeezingAxes:) does not have a derivative defined in the standard library.