[mlir] Add default implementations for methods in `TilingInterface`.

[mravishankar]

Summary

Adding the default implementation of getLoopIteratorTypes and
getLoopBounds allows ExternalModels to override these methods.

[nicolasvasilache]

Thanks! Landing for you so I can reuse.

mlir/include/mlir/Interfaces/TilingInterface.td

let methods = [
         /*desc=*/[{
           Returns a list of `StringRef`s that describe the number of
           loops and the iterator types of the operation. The list is
           expected to use
           `getParallelIteratorTypeName()`/`getReductionIteratorTypeName()`
           from MLIR Structured Op Utils.
         }],
         /*retType=*/"SmallVector<StringRef>",
-        /*methodName=*/"getLoopIteratorTypes"
+        /*methodName=*/"getLoopIteratorTypes",
+        /*args=*/(ins),
+        /*methodBody=*/"",
+        /*defaultImplementation=*/"return {};"
       >,
       InterfaceMethod<
         /*desc=*/[{
           Returns a list of ranges that describe the loop bounds and
           step for the loops of the operation.
         }],
         /*retTy=*/"SmallVector<Range>",
         /*methodName=*/"getLoopBounds",
-        /*args=*/(ins "OpBuilder &":$b)
+        /*args=*/(ins "OpBuilder &":$b),
+        /*methodBody=*/"",
+        /*defaultImplementation=*/"return {};"
       >,
       InterfaceMethod<
         /*desc=*/[{
           Method to generate the tiled implementation of an operation.
 
           The iteration space of the operation is returned by
           `getLoopBounds`. The caller provides the information of the
           tile within this iteration space whose implementation the