Rework clang-format include groups.

[ysiraichi]

This PR modifies clang-format configuration file, so that its include groups are tailored for PyTorch/XLA. In summary, the changes make it so includes in C++ files are grouped like:

/* System C Headers */
#include <unistd.h>

/* System C++ Headers */
#include <vector>

/* 
 * External Headers
 * - PyTorch Headers (e.g. torch/, c10/, and ATen/)
 * - abseil-cpp Headers
 * - OpenXLA Headers
 * 
 * Note that PyTorch headers should be included with angle-brackets so as to
 * keep consistency with PyTorch source code. This should also prevent a
 * header to be included twice.
 */
#include <torch/csrc/lazy/core/ir.h>
#include <c10/core/Scalar.h>
#include <ATen/ATen.h>
#include "absl/log/absl_check.h"
#include "xla/shape.h"

/* 
 * PyTorch/XLA Project Headers
 * - Source Headers
 * - Test Headers
 */
#include "torch_xla/csrc/tensor.h"
#include "test/cpp/cpp_test_util.h"

Key Changes:

• Modified .clang-format so as to recognize the include groups mentioned above
• Formatted all C++ files in the project
• Made all PyTorch includes use angle-brackets

Update (Nov 18):

• Merged "PyTorch Headers" with "External Headers"
• Merged "Test Headers" with "PyTorch/XLA Source Headers"

Update (Nov 18):

• Extract "PyTorch Headers" from "External Headers", making it into its own include group
  • Both PyTorch c10/util/logging_is_not_google_glog.h and abseil-cpp absl/log/log.h define a LOG() macro
  • Some include files from OpenXLA call that macro: if LOG() is redefined by PyTorch (i.e. it's included after abseil-cpp log file), and any of those OpenXLA files are included afterwards, they will use PyTorch LOG() definition rather than abseil-cpp. Raising the error.

[zhanyong-wan]

Please conform to the Google style guide for include ordering: https://google.github.io/styleguide/cppguide.html#Names_and_Order_of_Includes

In particular:

In dir/foo.cc or dir/foo_test.cc, whose main purpose is to implement or test the stuff in dir2/foo2.h, order your includes as follows:

dir2/foo2.h.
A blank line
C system headers, and any other headers in angle brackets with the .h extension, e.g., <unistd.h>, <stdlib.h>, <Python.h>.
A blank line
C++ standard library headers (without file extension), e.g., <algorithm>, <cstddef>.
A blank line
Other libraries' .h files.
A blank line
Your project's .h files.

Separate each non-empty group with one blank line.

Within each section the includes should be ordered alphabetically.

[ysiraichi]

I had to move PyTorch headers to their own group, right after the C++ system libraries. Otherwise, we would get a build error where some OpenXLA files would call the LOG() macro defined by PyTorch, rather than the one defined by abseil-cpp. See the comment above for more details.