GitLab

Summary:
extracted blob loader
added documentation for blob_loader
did some refactoring on fields
for detailed steps and discussions see:
https://github.com/facebookresearch/pytorch3d/pull/1463
https://github.com/fairinternal/pixar_replay/pull/160

Reviewed By: bottler

Differential Revision: D44061728

fbshipit-source-id: eefb21e9679003045d73729f96e6a93a1d4d2d51