I tried clamping the output post log softmax, this is the log generated
root - WARNING - Loss: 7753.49169921875
root - WARNING - Loss: 6186.9287109375
root - WARNING - Loss: 5434.07861328125
root - WARNING - Loss: 6422.82568359375
root - WARNING - Loss: 6344.4873046875
root - WARNING - Loss: 5779.78515625
root - WARNING - Loss: 5681.9140625
root - WARNING - Loss: 5288.10498046875
root - WARNING - Loss: 5314.443359375
root - WARNING - Loss: 4506.3115234375
root - WARNING - Loss: 5896.3134765625
root - WARNING - Loss: 6842.0830078125
root - WARNING - Loss: 9111.4599609375
root - WARNING - Loss: 7685.61328125
root - WARNING - Loss: 8802.61328125
root - WARNING - Loss: 11280.5126953125
root - WARNING - Loss: 14238.529296875
root - WARNING - Loss: 13673.314453125
root - WARNING - Loss: 13150.68359375
root - WARNING - Loss: 13360.0
root - WARNING - Loss: 13180.0
root - WARNING - Loss: nan

The NLLLoss becomes nan after few batches