MLBEDSW-8102: Fix regression on Argmax int64

- Fixed a regression where DepthWiseConv used in argmax int64
had the wrong shape.

- The error was introduced when adding support for a new operator
that changed the weight shape for the cast utility function. That
change only worked because reorder_depthwise_weights was called
later. Since argmax is converted after reorder_depthwise_weights
the cast operator in argmax got the wrong shape.

- The fix is to set the correct weight shape in the cast operator
and then mark that the weights already have been transposed correctly.

Change-Id: I61f5694f078cfcaf0d46d43faead6eb7e0a23ade
Signed-off-by: Johan Alfven <johan.alfven@arm.com>