Fix broken _assert_no_grad import

pytorch_binding/warpctc_pytorch/__init__.py

@@ -2,11 +2,16 @@
 import warpctc_pytorch as warp_ctc
 from torch.autograd import Function
 from torch.nn import Module
-from torch.nn.modules.loss import _assert_no_grad
 
 from ._warp_ctc import *
 
 
+def _assert_no_grad(tensor):
+    assert not tensor.requires_grad, \
+        "gradients only computed for acts - please " \
+        "mark other tensors as not requiring gradients"
+
+
 class _CTC(Function):
     @staticmethod
     def forward(ctx, acts, labels, act_lens, label_lens, size_average=False,