Fix pre-commit hook failures (#756)

Pre-commit hook fails when run on all files for three reasons:
(see trace below)

1. Trailing whitespace on multiple files
2. mypy fails to load numpy and then subsequently fails to load
LazyModule from pipe.py
3. isort sees issues with known_third_party packages

```
> pre-commit run --all-files

Trim Trailing Whitespace.................................................Failed
- hook id: trailing-whitespace
- exit code: 1
- files were modified by this hook

Fixing docs/source/conf.py
Fixing fairscale/experimental/nn/auto_shard.py
Fixing docs/source/deep_dive/activation_checkpointing.rst
Fixing docs/source/tutorials/pipe.rst
Fixing docs/source/installation_instructions.rst
Fixing docs/source/deep_dive/pipeline_parallelism.rst
Fixing docs/source/tutorials/activation_checkpointing.rst
Fixing docs/source/tutorials/offload_model.rst
Fixing docs/source/deep_dive/oss_sdp_fsdp.rst
Fixing docs/source/what_is_fairscale.rst
Fixing CHANGELOG.md
Fixing fair...

pyproject.toml

@@ -27,4 +27,4 @@ use_parentheses = true
 skip_glob = ["build/*", "stubs/*"]
 # Don't split "import" and "from".
 force_sort_within_sections = true
-known_third_party = ["benchmark_dataset", "datasets", "golden_configs", "helpers", "models", "numpy", "parameterized", "pytest", "recommonmark", "setuptools", "torch", "torch_pg", "torchtext", "torchvision"]
+known_third_party = ["benchmark_dataset", "datasets", "golden_configs", "models", "numpy", "parameterized", "pytest", "recommonmark", "setuptools", "torch", "torchtext", "torchvision"]