[BACKEND][NFC] Interface for LinearLayout conversion. (#4525)

Currently, implementing a LinearLayout conversion from a new layout
requires implementing the conversion method in
`LinearLayoutConversions.cpp`.
The current approach is not ideal because:
- It does not support modular design in that a third-party layout cannot
implement its own conversion externally.
- There is no compile-time method to check whether a layout supports
LinearLayout conversion.

This PR suggests adding a TableGen trait to interface LinearLayout
conversion method.
The process is now simplified as 1) add `LinearLayoutTrait` to the
layout and implement`[Layout]::toLinearLayout` function.


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

- [x] I have written a PR description following these
  [rules](https://cbea.ms/git-commit/#why-not-how).

- [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 it is an improvement of
programming interface of an existing feature.

- Select one of the following.
  - [x] I have not added any `lit` tests.
- [ ] The `lit` tests I have added follow these [best
practices](https://mlir.llvm.org/getting_started/TestingGuide/#filecheck-best-practices),
including the "tests should be minimal" section. (Usually running Python
code
    and using the instructions it generates is not minimal.)

include/triton/Dialect/TritonGPU/IR/TritonGPUAttrDefs.td

@@ -525,6 +525,10 @@ For the Threads Per Warp and Values Per Thread level, the linear id distribution
     InterfaceMethod<"Gets the number of contiguous elements per thread.",
                     "SmallVector<unsigned>",
                     "getContigPerThread">,
+    InterfaceMethod<"Convert to LinearLayout.",
+                    "std::optional<LinearLayout>",
+                    "toLinearLayout",
+                    (ins "ArrayRef<int64_t>":$shape)>
   ];
 }
 
@@ -576,6 +580,8 @@ L(T) = [ {0,8} , {1,9} , {2,10}, {3,11}, {0,8} , {1, 9} , {2, 10}, {3, 11},
 
     SmallVector<unsigned> getSizePerThread() const;
     SmallVector<unsigned> getShapePerCTATile(ArrayRef<int64_t> tensorShape = ArrayRef<int64_t>()) const;
+
+    std::optional<LinearLayout> toLinearLayout(ArrayRef<int64_t> shape) const;
   }];
 }
 

include/triton/Dialect/TritonGPU/IR/TritonGPUInterfaces.h

@@ -1,6 +1,6 @@
 #ifndef TRITON_GPU_DIALECT_INTERFACES_H
 #define TRITON_GPU_DIALECT_INTERFACES_H
-
+#include "triton/Dialect/TritonGPU/IR/LinearLayoutConversions.h"
 #include "triton/Dialect/TritonGPU/IR/TritonGPUAttrInterfaces.h.inc"
 
 #endif // TRITON_GPU_DIALECT_INTERFACES_H