*_pool*d: improve error handling and error messages.

[ysiraichi]

This PR refactors {avg,max}_pool{2,3}d operation implementations by improving their error message, and returning a status type value.

Key Changes:

• Make tensor_methods::avg_pool_nd{,_backward} return StatusOr<XLATensorPtr>
• Make tensor_methods::max_pool_nd{,_backward} return StatusOr<std::tuple<XLATensorPtr, XLATensorPtr>>
• Improve error messages and error handling
  • Remove CheckIntList
  • Create the following new functions:
    • RepeatIfSingleElement(span, n): if span is a single-element list, create a new one repeating it n times. Otherwise return the elements in span.
    • CheckPoolNdInputHasSize(...): check that the given list has a specific size
    • FillAndCheckPoolNdInputs(...): runs the 2 functions above for *_pool*d common inputs, i.e. kernel_size, stride, and padding

Example

a = torch.rand(1, 1, 4, 4, 4, device=device)
kernel_size = [2, 2]
stride = []
padding = [0]
torch.nn.functional.avg_pool3d(a, kernel_size, stride, padding)

Before:

Traceback (most recent call last):
  File "examples/pool.py", line 26, in <module>
    torch.nn.functional.avg_pool3d(a, kernel_size, stride, padding)
RuntimeError: Check failed: result.size() == length (2 vs. 3)Invalid length for the 'kernel_size' attribute (at torch_xla/csrc/tensor_methods.cpp:267)

Exception raised from operator& at torch_xla/csrc/runtime/tf_logging.cpp:26 (most recent call first):

After:

Traceback (most recent call last):
  File "examples/pool.py", line 26, in <module>
    torch.nn.functional.avg_pool3d(a, kernel_size, stride, padding)
RuntimeError: avg_pool3d(): expected argument kernel_size [2, 2] (size: 2) to have size of 3.

Status Propagation Trace:
    From: CheckPoolNdInputHasSize at torch_xla/csrc/tensor_methods.cpp:275 (error: avg_pool3d(): expected argument kernel_size [2, 2] (size: 2) to have size of 3.)
    From: FillAndCheckPoolNdInputs at torch_xla/csrc/tensor_methods.cpp:292
    From: avg_pool_nd at torch_xla/csrc/tensor_methods.cpp:1257
    From: avg_pool3d at torch_xla/csrc/aten_xla_type.cpp:1228

Exception raised from ThrowStatusError at torch_xla/csrc/status.cpp:128 (most recent call first):