Fix issues from merge of PR #254

[alexhernandezgarcia]

Fixes two issues from the merge of PR https://github.com/alexhernandezgarcia/gflownet/pull/253:

• The input to the proxy must, in general, be a batch tensor, not a single state.
• One test used torch.equal but did fail most times. Changed to torch.all(torch.isclose(...)).