RuntimeError: element 0 of tensors does not require grad and does not have a grad_fn

[ch3njust1n]

Process Process-5:1:2:
Traceback (most recent call last):
  File "/home/ubuntu/anaconda2/envs/smpl/lib/python2.7/multiprocessing/process.py", line 267, in _bootstrap
    self.run()
  File "/home/ubuntu/anaconda2/envs/smpl/lib/python2.7/multiprocessing/process.py", line 114, in run
    self._target(*self._args, **self._kwargs)
  File "/home/ubuntu/smpl/distributed/train.py", line 65, in train
    loss.backward()
  File "/home/ubuntu/anaconda2/envs/smpl/lib/python2.7/site-packages/torch/tensor.py", line 93, in backward
    torch.autograd.backward(self, gradient, retain_graph, create_graph)
  File "/home/ubuntu/anaconda2/envs/smpl/lib/python2.7/site-packages/torch/autograd/__init__.py", line 90, in backward
    allow_unreachable=True)  # allow_unreachable flag
RuntimeError: element 0 of tensors does not require grad and does not have a grad_fn