Change ConvPoolOp<Context>::SetOutputSize to ConvPoolOp<Context>::GetOutputSize ...

Change ConvPoolOp<Context>::SetOutputSize to ConvPoolOp<Context>::GetOutputSize (#16273)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/16273

Previously we have SetOutputSize which accept a partially initialized Output Tensor and set it to the correct size,
the diff change this to GetOutputSize that returns the correct size instead.
e.g.
```
auto* Y = Output(0);
ConvPoolOp<Context>::SetOutputSize(X, Y, channels);
...
Y->mutable_data<T>...
```
-->
```
auto sizes = ConvPoolOp<Context>::GetOutputSize(X, channels);
auto* Y = Output(0, sizes, at::dtype<T>());
```

Reviewed By: dzhulgakov

Differential Revision: D13736281

fbshipit-source-id: 64abce3dbaed0b375098463333dfd0ea5a3b1945