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Summary:
In opt mode, the linker could strip out the symbols related to the torchvision ops if they had not been explicitly requested.

Adding link-whole is one way of fixing this.

Reviewed By: datumbox, ppwwyyxx

Differential Revision: D26249713

fbshipit-source-id: 4e55e2ebb483922fe308fd8b83456439bde728c9

Summary:
This diff adds a new target to torchvision which enables users to use torchvision ops from C++.

For now, the `cpp_library` is not used by the `python_cpp_library`. We should instead refactor the logic in torchvision to directly use `cpp_library` instead.

There is currently an inconsistency between fbcode and OSS users. OSS users can import torchvision via
```
#include <torchvision/vision.h>
```
while fbcode users need to do
```
#include <torchvision/csrc/vision.h>
```
It would be good to fix this discrepancy in the future.

I didn't directly use `test_frcnn_tracing.cpp` due to complications for getting the `.pt` file in a way that works for both OSS and fbcode, so instead we added a self-contained test that should validate that the torchvision ops are properly registered and visible to JIT

Reviewed By: datumbox

Differential Revision: D26225669

fbshipit-source-id: 5dd9fb98dd58e854f95806e4860d02f54fc04ea4
Co-authored-by: Francisco Massa <fmassa@fb.com>