Moved from fvcore's PathManager to iopath's

[thomasehuang]

This fixes recent issues about the demo that is caused by the fvcore update.

[lgtm-com]

This pull request fixes 1 alert when merging 34028a7 into 6b0769b - view on LGTM.com

fixed alerts:

• 1 for Unused import

[lgtm-com]

This pull request fixes 1 alert when merging 4944930 into 6b0769b - view on LGTM.com

fixed alerts:

• 1 for Unused import

[Anthuang]

Still seeing the following when running the demo:

** fvcore version of PathManager will be deprecated soon. **
** Please migrate to the version in iopath repo. **
https://github.com/facebookresearch/iopath

Should change these too:

https://github.com/ucbdrive/few-shot-object-detection/blob/model_zoo/tools/visualize_json_results.py#L7

https://github.com/ucbdrive/few-shot-object-detection/blob/model_zoo/datasets/prepare_voc_few_shot.py#L2