Fix padding

Author: Alejandro Gaston Alvarez Franceschi

Diff for: coremltools/converters/mil/mil/passes/defs/lower_complex_dialect_ops.py

@@ -482,8 +482,9 @@ def _istft(
     # We need to adapt last dimension
     if length is not None:
         if length > expected_output_signal_len:
-            real_result = mb.pad(x=real_result, pad=, mode="constant", constant_val=0, before_op=before_op)
-            imag_result = mb.pad(x=imag_result, pad=, mode="constant", constant_val=0, before_op=before_op)
+            right_pad = mb.fill(shape=(channels, expected_output_signal_len - length), value=0., before_op=before_op)
+            real_result = mb.stack(x=(real_result, right_pad), axis=1, before_op=before_op)
+            imag_result = mb.stack(x=(imag_result, right_pad), axis=1, before_op=before_op)
         elif length < expected_output_signal_len:
             real_result = mb.slice_by_size(x=real_result, begin=[0], size=[length], before_op=before_op)
             imag_result = mb.slice_by_size(x=imag_result, begin=[0], size=[length], before_op=before_op)