[skip ci][frontend] Add support to AG for Jax single array assignment

frontend/catalyst/ag_primitives.py

@@ -44,6 +44,7 @@
     "and_",
     "or_",
     "not_",
+    "set_item",
 ]
 
 
@@ -565,6 +566,31 @@ def qnode_call_wrapper():
         return ag_converted_call(fn, args, kwargs, caller_fn_scope, options)
 
 
+def set_item(target, i, x):
+    """An implementation of the AutoGraph 'set_item' function. The interface is defined by
+    AutoGraph, here we merely provide an implementation of it in terms of Catalyst primitives.
+    The idea is simply to accept the much simpler single index assigment syntax for Jax arrays, 
+    to subsequently transform it under the hood into the set of 'at' and 'set' calls that
+    Autograph supports. E.g.:
+        target[i] = x -> target = target.at[i].set(x)
+
+    .. note::
+        For this feature to work, 'converter.Feature.LISTS' had to be added to the 
+        TOP_LEVEL_OPTIONS and NESTED_LEVEL_OPTIONS conversion options of our own Catalyst 
+        Autograph transformer. If you create a new transformer and want to support this feature, 
+        make sure you enable such option there as well.
+    """
+
+    # Apply the 'at...set' transformation only to Jax arrays.
+    # Otherwise, fallback to Python's default syntax.
+    if isinstance(target, DynamicJaxprTracer):
+        target = target.at[i].set(x)
+    else:
+        target[i] = x
+
+    return target
+
+
 class CRange:
     """Catalyst range object.
 

frontend/catalyst/autograph.py

@@ -265,18 +265,21 @@ def __init__(self):
 # Singleton instance of DisableAutograph
 disable_autograph = DisableAutograph()
 
+# converter.Feature.LISTS permits overloading the 'set_item' function in 'ag_primitives.py'
+OPTIONAL_FEATURES = [converter.Feature.BUILTIN_FUNCTIONS, converter.Feature.LISTS]
+
 TOPLEVEL_OPTIONS = converter.ConversionOptions(
     recursive=True,
     user_requested=True,
     internal_convert_user_code=True,
-    optional_features=[converter.Feature.BUILTIN_FUNCTIONS],
+    optional_features=OPTIONAL_FEATURES,
 )
 
 NESTED_OPTIONS = converter.ConversionOptions(
     recursive=True,
     user_requested=False,
     internal_convert_user_code=True,
-    optional_features=[converter.Feature.BUILTIN_FUNCTIONS],
+    optional_features=OPTIONAL_FEATURES,
 )
 
 STANDARD_OPTIONS = converter.STANDARD_OPTIONS