* adding registry to hook custom building blocks

* adding customizable rpn head

* support customizable c2 weight loading

* added Dockerfile

* intructions for installation with docker

* adds nccl to dockerfile

* Update Dockerfile

* Update INSTALL.md

## Bug
When I repeatedly invoke build_detection_model function to build the model, I will get the error: `AttributeError: 'ResNet' object has no attribute 'layer1'`. 

## To Reproduce
```
model = build_detection_model(cfg)
model = build_detection_model(cfg)
```

## The reason
The variable `ResNet50StagesTo4` is a global generator expression, so when we build the model secondly, the code executes to Line 82 `for stage_spec in stage_specs:` and `stage_specs` will return empty leading to do not add any stage. Finally, `self._freeze_backbone` try to freeze the backbone by executing `m = getattr(self, "layer" + str(stage_index))` firstly. At the moment, it will throw the AttributeError `AttributeError: 'ResNet' object has no attribute 'layer1'`.

I guess you want to define ResNet50StagesTo4 as the tuple, so I try to fix by add tuple type qualifier.

## The solution
Add the tuple type to `ResNet50StagesTo5`, `ResNet50StagesTo4`, `ResNet50FPNStagesTo5`, `ResNet101FPNStagesTo5`.

I do not know whether there are similar bug existing, so you need to review my solution. Thank you!

* quick fix for area

* Update bounding_box.py

use cuda version torch.randperm to avoid copy from gpu to cpu and a fatal bug in multi-thread cpu version

Replace BoxList argument size with image_size.

Replace baseline with benchmark

Fix BoxList constructor call in README.md

* Add missing __init__.py files

* Add packages

* Rename logging.py to logger.py

Import rules from Python2 makes this a bad idea

* Make import_file py2 compatible

* list does not have .copy() in py2

* math.log2 does not exist in py2

* Miscellaneous fixes for py2

* Address comments

Access repos via https in INSTALL.md

It is simpler and requires less effort (i.e. no auth) compared to SSH

Fix a small typo

Add ISSUE_TEMPLATE for Github

Update INSTALL.md

* Update .gitignore

* Update .gitignore