Warning when copying a tensor in the base sampler

[yukw777]

Describe the bug
The base sampler produces the following warning:

py.warnings [block_train] /home/ubuntu/.local/lib/python3.6/site-packages/flambe/sampler/base.py:167: UserWarning: To copy construct from a tensor, it is recommended to use sourceTensor.clone().detach() or sourceTensor.clone().detach().re
quires_grad_(True), rather than torch.tensor(sourceTensor).
  tensors = [torch.tensor(example) for example in column]

To Reproduce
Steps to reproduce the behavior:
Simply run the base sampler with verbose logging.

Expected behavior
There should be no warning.

Screenshots
N/A

Software Versions (please complete the following information):

• OS: Ubuntu 18.04
• Python Version 3.6.8
• PyTorch Version 1.1
• Flambé Version 0.4.8

Additional context
N/A