Resolve unsoundness caught by pytype --strict-none-binding.

Pytype was previously more permissive when variables were initialized to
None but may have been re-assigned later. This change improves pytype's ability to catch unsoundness in the affected files.

PiperOrigin-RevId: 538126469
Change-Id: I7d440de891f09d1de9d50d5522209d4ddff915cb

Author: Googler

tensorflow_compression/python/layers/signal_conv.py

@@ -653,7 +653,7 @@ def _correlate_down_valid(self, inputs, kernel):
           strides=strides, padding="VALID", data_format=data_format)
       # Perform remaining downsampling.
       slices = tuple(slice(None, None, s // gcf) for s in self.strides_down)
-      if any(s.step > 1 for s in slices):
+      if any(s.step > 1 for s in slices):  # pytype: disable=unsupported-operands
         outputs = outputs[self._padded_tuple(slices, slice(None))]
     else:
       self._raise_notimplemented()