Cannot do mul for int tensors

Hi, @dcslin , we cannot do the mul operator for int tensors:

The error is:
> F0324 05:04:22.542809 14739 tensor.cc:932] Unknown combination of data type kInt and language kCuda

please use this test case:
```python3
x1 = np.array([1], dtype=np.int32)
x2 = np.array([256], dtype=np.int32)
x1 = tensor.from_numpy(x1)
x1.to_device(gpu_dev)
x2 = tensor.from_numpy(x2)
x2.to_device(gpu_dev)

y = autograd.Mul()(x1, x2)
print(tensor.to_numpy(y[0]))
```