Feature: einsum

Good day!
Could you please Implement something like numpy.einsum (for OpenCL and CUDA)?

In particular, I'm interested in row/column-wise outer product, like this, but einsum is very universal and useful thing...

```
A = np.random.randint(1,7, size=(3,2))
B = np.random.randint(1,7, size=(2,3))
X = np.einsum('ik,kj->kij', A,B)
X
array([[[ 8, 16, 12],
[ 8, 16, 12],
[ 4, 8, 6]],

[[15, 3, 12],
[ 5, 1, 4],
[ 5, 1, 4]]])
```