Fix some sign issues (#68361)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/68361
Fixes
```
caffe2/aten/src/ATen/FunctionalizeFallbackKernel.cpp:36:31: error: comparison of integers of different signs: 'int64_t' (aka 'long') and 'const unsigned long' [-Werror,-Wsign-compare]
for (int64_t idx = 0; idx < num_returns; ++idx) {
~~~ ^ ~~~~~~~~~~~
caffe2/aten/src/ATen/native/cuda/Sorting.cpp:87:16: error: comparison of integers of different signs: 'int64_t' (aka 'long') and 'std::vector::size_type' (aka 'unsigned long') [-Werror,-Wsign-compare]
assert(dim < out_shape.size());
~~~ ^ ~~~~~~~~~~~~~~~~
```
Test Plan: Sandcastle
Reviewed By: malfet
Differential Revision: D32433063
fbshipit-source-id: b896dbab81861f3f074e00db73d20d9523037dd1
