Support conversion from Caffe2 MergeDim to ONNX Reshape + Squeeze. (#16189)
Summary:
`MergeDim` can be done by `Reshape([1, -1, 0, 0, ...]) + Squeeze`.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16189
Differential Revision: D14070676
Pulled By: ezyang
fbshipit-source-id: 28d7e9b35cc2c1dcbd4afb3fbdf7383e219b1777
