Allow Average Pooling modules to accept tensors with 0-dim batch sizes. (#62025)

Allow Average Pooling modules to accept tensors with 0-dim batch sizes. (#62025)

Summary:
This issue fixes a part of https://github.com/pytorch/pytorch/issues/12013, which is summarized concretely in  https://github.com/pytorch/pytorch/issues/38115.

It introduces changes and tests for allowing the Average Pooling layers to accept tensors with 0 sized batch dimensions and return meaningful results.

Pull Request resolved: https://github.com/pytorch/pytorch/pull/62025

Reviewed By: VitalyFedyunin

Differential Revision: D30303256

Pulled By: jbschlosser

fbshipit-source-id: 5f727e62a7c58d2b8bb49fcc3bd7688474917ba5