[Fix] Fix build with cuda9.0

[grimoire]

Motivation

Can not build mmcv-full with cuda9.0.

Reference
pytorch/extension-cpp#35

Modification

Replace torch/extension.h with torch/types.h

[teamwong111]

Checked.

[ZwwWayne]

shall we leave a note for that in case we cause the similar bug in the future？

[grimoire]

@ZwwWayne sure. Where should I put the note? In code?

[grimoire]

By the way, I prefer <torch/extension.h> since this seems like the official way to add custom ops.
https://pytorch.org/tutorials/advanced/cpp_extension.html

[ZwwWayne]

We can add a comment just in the line above line 17. I also thought we should use torch.extension.h, so I think we should notify other developers in the code. An example can be as below:

Using <torch/extension.h> is recommended in the official documentation in https://pytorch.org/tutorials/advanced/cpp_extension.html#writing-the-c-op.
However, we use torch/types.h for compatibility with CUDA 9.0