[Relax] Implement relax.transform.TopologicalSort (#16697)

* [Relax] Implement relax.transform.TopologicalSort

This commit implements a utility `relax.transform.TopologicalSort`,
which can re-order the bindings that occur in a
`relax.DataflowBlock`.  This is not intended for use in a
general-purpose optimization pipeline, but instead as a utility that
may be used as needed in specific cases.  For example, normalization
of unit tests that should not depend on the order of variable binding.

* Update docstring according to review comment

python/tvm/relax/transform/__init__.py

@@ -72,6 +72,7 @@
     StaticPlanBlockMemory,
     ToMixedPrecision,
     ToNonDataflow,
+    TopologicalSort,
     UpdateParamStructInfo,
     UpdateVDevice,
     VMBuiltinLower,

python/tvm/relax/transform/transform.py

@@ -233,6 +233,29 @@ def ToNonDataflow() -> tvm.ir.transform.Pass:
     return _ffi_api.ToNonDataflow()  # type: ignore
 
 
+def TopologicalSort(order="depth-first", direction="from-inputs") -> tvm.ir.transform.Pass:
+    """Sort bindings in relax.Dataflow blocks in the order specified
+
+    Parameters
+    ----------
+    order: str
+
+        The order in which bindings should be emitted.  Allowed values
+        are "depth-first" and "breadth-first".
+
+    direciton: str
+
+        The direction in which the sort should be performed.  Allowed
+        values are "from-inputs" and "from-outputs".
+
+    Returns
+    -------
+    ret: tvm.ir.transform.Pass
+
+    """
+    return _ffi_api.TopologicalSort(order, direction)  # type: ignore
+
+
 def RemovePurityChecking() -> tvm.ir.transform.Pass:
     """Activate relax.force_pure on all pure functions in the module
     and unwrap all pure override ops into the normal versions.