[Build] Allow out-of-tree build with TRITON_BUILD_DIR (#7347)

# Rationale

Current Triton uses a fixed build directory
`f"cmake.{plat_name}-{sys.implementation.name}-{python_version}"` when
installing with `pip install .`, when the same python is used,
regardless of the venv target or source code version. This creates
conflicts when

* Users switches Triton source version but did not clean the build
directory.
+ This is a little hard to notice since most `pip install .` will use a
temporary directory to build
* Users installing Triton into multiple venv at the same time.

With this option, users can specify an out-of-tree build directory to
avoid such conflicts.

# Alternatives

PIP usually uses /tmp/pip-build-env-xxxx/ as the temporary build
directory. However I have not found documentation about how to locate
this directory in `pip/setuptools`.

Author: xinyazhang

python/build_helpers.py

@@ -8,10 +8,15 @@ def get_base_dir():
     return os.path.abspath(os.path.join(os.path.dirname(__file__), os.pardir))
 
 
-def get_cmake_dir():
+def _get_cmake_dir():
     plat_name = sysconfig.get_platform()
     python_version = sysconfig.get_python_version()
     dir_name = f"cmake.{plat_name}-{sys.implementation.name}-{python_version}"
-    cmake_dir = Path(get_base_dir()) / "build" / dir_name
+    return Path(get_base_dir()) / "build" / dir_name
+
+
+def get_cmake_dir():
+    cmake_dir = os.getenv("TRITON_BUILD_DIR", default=_get_cmake_dir())
+    cmake_dir = Path(cmake_dir)
     cmake_dir.mkdir(parents=True, exist_ok=True)
     return cmake_dir