Unify all pallas kernel code in one directory

[miladm]

🐛 Bug

Please move https://github.com/pytorch/xla/blob/master/torch_xla/experimental/custom_kernel.py to the https://github.com/pytorch/xla/blob/master/torch_xla/experimental/pallas_kernels directory to unify all kernel code in one location.

Let's do the same for the test codes under /test director.

[vanbasten23]

https://github.com/pytorch/xla/blob/master/torch_xla/experimental/custom_kernel.py is not a Pallas kernel. Today, everything under https://github.com/pytorch/xla/blob/master/torch_xla/experimental/pallas_kernels are Pallas kernel. So it seems to me that it makes more sense to keep custom_kernel.py at the current location.