Summary:
Adding support for type checking using mypy.
Cleaning up a bunch of files to fix existing type check errors.
Note that for now we are only type checking non-FLAVA code inside torchmultimodal.
Also adding some instructions to CONTRIBUTING.md

Test plan:
```
mypy
Success: no issues found in 28 source files
```

```
python3 -m pytest -vv
...
42 passed, 2 skipped, 26 warnings in 22.87s
```

[ghstack-poisoned]

Summary:
Adding support for type checking using mypy.
Cleaning up a bunch of files to fix existing type check errors.
Note that for now we are only type checking non-FLAVA code inside torchmultimodal.
Also adding some instructions to CONTRIBUTING.md

Test plan:
```
mypy
Success: no issues found in 28 source files
```

```
python3 -m pytest -vv
...
42 passed, 2 skipped, 26 warnings in 22.87s
```

[ghstack-poisoned]