Add Operators: subtraction, multiplication, concat and stride slice

[dboyliao]

Add operators to runtime:

• SubOperator: elementwise subtraction
• MulOperator: elementwise multiplication
• ConcatOperator: concat tensors along given axis
• StrideSliceOperator: strided tensor slicing, ex t[:, ..., :3]

[mbartling]

@dboyliao Can you add some quick comments on the StridedIterator in this chat? I'm pretty sure you wrote the original stride op

[dboyliao]

Ok.
The basic idea of StridedIterator is simple, it will give you the linear index/offset of the source tensor given the stride slice specs.
For example, if the given slice specs are 0:3 and 1:5:2 and input tensor of shape (5, 10), all possible strided indices are:

• (0, 1), with linear index 1
• (0, 3), with linear index 3
  ....
• (2, 9), with linear index 29

Then in the strided slice op, I only need to copy value from source tensor based on the linear indices.