Update _SimpleConsensus to use static autograd methods (for PyTorch >1.3)

[tchang1997]

Thank you so much for sharing your implementation of TPN!

Problem

I've been trying to get it to work in one of my own projects -- however, I ran into the same issue as mentioned #28, in which the user pastes a stack trace with error message "Legacy autograd function with non-static forward method is deprecated." This occurs when you try to call forward() with the old code when the averaging consensus (_SimpleConsensus) is used.

Environment

• PyTorch 1.7.0 + CUDA 11.0
• Ubuntu 16.04.2 LTS
• Python 3.7.8

Summary of changes

In order to make the _SimpleConsensus class (subclassing torch.Autograd.Function) compatible with PyTorch >1.3:

• Removed __init__ method from _SimpleConsensus
• Use apply static method instead of forward for passing input tensor through the _SimpleConsensus object
• forward() method of _SimpleConsensus uses ctx.save_for_backward(args) to cache input tensor x, dim, and consensus_type.
• self.shape is no longer a member of _SimpleConsensus; it is reconstructed by retrieving x from ctx.saved_tensors and calling x.size() in each call to backward().

This is consistent with the template given in the PyTorch docs, which I referenced.

Discussion

The changes in this PR work for me -- I am able to run forward() without issue now. However, as a disclaimer, due to the nature of my project, I'm using my own testing script instead of the provided testing framework in this repo. For completeness, my model loading code looks like this:

from TPN.mmaction.models.recognizers import TSN3D
import torch

PRETRAINED_MODEL_PATH = "/path/to/my/model/kinetics400_tpn_r50f32s2.pth"
model = TSN3D(model_cfg["backbone"], necks=model_cfg["necks"], cls_head=model_cfg["cls_head"],
             spatial_temporal_module=model_cfg["spatial_temporal_module"],
              segmental_consensus=model_cfg["segmental_consensus"])
pretrained = torch.load(PRETRAINED_MODEL_PATH)
model.load_state_dict(pretrained)

Please let me know if there's any additional testing (suites or otherwise) I should run, or if there's a contributing guide that I've overlooked. Furthermore, I'm happy to provide more details as needed. Thanks!

[shreyas-bk]

It's saying ctx is not defined for me, am I missing something?

[tchang1997]

Missed a typo; thanks for pointing that out. I overwrote my previous changes on my branch. Does it work for you now?

[shreyas-bk]

The ctx not defined error is solved, but there's a new error. It seems like save_for_backward can only save tensors, not dim(int) and consensus_type(str): TypeError: save_for_backward can only save variables, but argument 1 is of type int

[shreyas-bk]

I've created a PR on your fork with the code that works for me: PR