[FRONTEND] Enable triton to support register thirdparty backend at runtime (#1643)

This PR intends to provide a mechanism to support a third-party backend
at runtime to generate the backend-specific code.

The mechanism provided a common class to abstract the third-party
backend logic and two essential functions to register and get the
third-party backend at runtime.

- `BaseBackend`: A common class to abstract the third-party backend
logic
- `register_backend`: Register a third-party backend with a given device
type
- `get_backend`: Get the third-party backend with a given device type

Generally, a third-party backend must inherit from `BaseBackend` and
implement all the member functions according to the backend
characteristics. As long as the backend implementation is ready, the
third-party backend can invoke `register_backend` to register it under a
given device. During the kernel compilation and execution, the mechanism
will get the registered backend to generate the kernel and launcher code
for a given device.

This PR added a dummy backend to simulate a third-party backend and
demonstrate the usage.

-
[test_device_backend.py](https://github.com/openai/triton/pull/1643/files#diff-bbe4d50624f2d11bf17c878a1ed4d422918c124c182cf9357b993240c385bea1):
To define a third-party backend and register the backend
-
[ExtensionBackend](https://github.com/openai/triton/pull/1643/files#diff-bbe4d50624f2d11bf17c878a1ed4d422918c124c182cf9357b993240c385bea1R123):
Inherit from the `BaseBackend` and implement some specific logic like
[filter out some compile
stages](https://github.com/openai/triton/pull/1643/files#diff-bbe4d50624f2d11bf17c878a1ed4d422918c124c182cf9357b993240c385bea1R129-R135)
- [Register the `ExtensionBackend` for
`CPU`](https://github.com/openai/triton/pull/1643/files#diff-bbe4d50624f2d11bf17c878a1ed4d422918c124c182cf9357b993240c385bea1R279)
  
-
[extension_backend.c](https://github.com/openai/triton/pull/1643/files#diff-169c1d08b3a0a7b343cfa3258fbc32b47e0f6c46305a112652fa1bdaaec89d29):
To provide the utility function to load kernel binary and get the
backend properties.

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "third_party/intel_xpu_backend"]
+	path = third_party/intel_xpu_backend
+	url = http://github.com/intel/intel-xpu-backend-for-triton