remove the slow path(NCHW) for avg_pool3d (#34994)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/34994
Use the fast path for NCHW input tensor
Test Plan: run the pool unit tests
Differential Revision: D20522082
fbshipit-source-id: 6e834425d06fbb1a105d851c2c36ef73df9de08f
