[fx2trt] prod dim is not wrapped as a tuple for single dim

Summary:
as titled
CI test did not caught this failure before

Reviewed By: wushirong

Differential Revision: D34932995

fbshipit-source-id: 822ce98fde0545835af3443150275546a946691d

Author: Wei Wei

fx/converters/converter_utils.py

@@ -523,6 +523,9 @@ def add_reduce_layer(
     else:
         dim = kwargs["dim"]  # type: ignore[assignment]
 
+    if not isinstance(dim, Sequence):
+        dim = (dim,)
+
     if not network.has_implicit_batch_dimension:
         dim = tuple(len(input_val.shape) + i if i < 0 else i for i in dim)
     else: