[frontend] outline register_transform (#1705)

**Context:** I need to be able to register equivalence from plxpr
transforms to MLIR transforms. There is this piece of code that does
that, but it is not a function.

**Description of the Change:** Outline that piece of code into its own
function.

**Benefits:** I can register the equivalence from plxpr transforms to
MLIR transforms.

**Possible Drawbacks:** None

**Related GitHub Issues:** Fixes #1703

Author: erick-xanadu

doc/releases/changelog-dev.md

@@ -59,6 +59,9 @@
 
 <h3>Internal changes ⚙️</h3>
 
+* Creates a function that allows developers to register an equivalent MLIR transform for a given PLxPR transform.
+  [(#1705)](https://github.com/PennyLaneAI/catalyst/pull/1705)
+
 * Stop overriding the `num_wires` property when the operator can exist on `AnyWires`. This allows the deprecation
   of `WiresEnum` in pennylane.
   [(#1667)](https://github.com/PennyLaneAI/catalyst/pull/1667)

frontend/catalyst/from_plxpr.py

@@ -201,11 +201,10 @@ def handle_qnode(
 }
 
 
-# This is our registration factory for PL transforms. The loop below iterates
-# across the map above and generates a custom handler for each transform.
-# In order to ensure early binding, we pass the PL plxpr transform and the
-# Catalyst pass as arguments whose default values are set by the loop.
-for pl_transform, (pass_name, decomposition) in transforms_to_passes.items():
+# pylint: disable-next=redefined-outer-name
+def register_transform(pl_transform, pass_name, decomposition):
+    """Register pennylane transforms and their conversion to Catalyst transforms"""
+
     # pylint: disable=unused-argument, too-many-arguments, cell-var-from-loop
     @WorkflowInterpreter.register_primitive(pl_transform._primitive)
     def handle_transform(
@@ -251,6 +250,14 @@ def wrapper(*args):
             return self.eval(inner_jaxpr, consts, *non_const_args)
 
 
+# This is our registration factory for PL transforms. The loop below iterates
+# across the map above and generates a custom handler for each transform.
+# In order to ensure early binding, we pass the PL plxpr transform and the
+# Catalyst pass as arguments whose default values are set by the loop.
+for pl_transform, (pass_name, decomposition) in transforms_to_passes.items():
+    register_transform(pl_transform, pass_name, decomposition)
+
+
 class QFuncPlxprInterpreter(PlxprInterpreter):
     """An interpreter that converts plxpr into catalyst-variant jaxpr.