[AMD] Improve codebase style consistency in third_party/amd

[knwng]

This commit improved codebase style consistency in 3 aspects

1. Updated namespace declaration to a nested fasion
2. Unified header guards
3. Updated comment styles following LLVM/MLIR style, focusing on the comments on the declarations of top-level function/class/struct and class members.

New contributor declaration

• [x] I am not making a trivial change, such as fixing a typo in a comment.

• [ ] I have written a PR description following these rules.

• [x] I have run pre-commit run --from-ref origin/main --to-ref HEAD.

• Select one of the following.

  • [ ] I have added tests.
  • /test for lit tests
  • /unittest for C++ tests
  • /python/test for end-to-end tests
  • [x] This PR does not need a test because lint changes.

• Select one of the following.

  • [x] I have not added any lit tests.
  • [ ] The lit tests I have added follow these best practices, including the "tests should be minimal" section. (Usually running Python code and using the instructions it generates is not minimal.)

[knwng]

For record, the code snippets this PR fixed was found by the following commands:

namespace:

cd third_party/amd/
grep -nrI --exclude-dir "backend" "namespace" .

header guard:

#!/bin/bash

set -euxo pipefail

ROOT=$(cd .. && pwd)

cd ./third_party/amd/

cpplint \
    --exclude="backend/" \
    --root=${ROOT} \
    --repository=${ROOT} \
    --recursive \
    --filter=-runtime,-whitespace,-legal/copyright,-readability/todo,-build/include_what_you_use,-build/include_order,-build/include,+build/namespaces,+build/header_guard,-build/namespaces \
    --quiet \
    .

comments:

cd third_party/amd/
egrep -nrI --exclude-dir "backend" "^\ +//\ " .

And since third_party/amd/backend/ has many files ported from other codebases, it has been excluded in this PR.