Merge output shape with input shape instead of override (#2578)

`_constant_folding.cast` override `output.shape` with `input.shape`,
that may make a static shape to dynamic shape. Here should use
`_merge_shapes` instead.

Author: wodesuck

onnxscript/optimizer/_constant_folding.py

@@ -501,9 +501,7 @@ def cast(node: ir.Node, op, state: OptimizerState) -> ReturnValue:
     # should handle this. Only the optimization to eliminate redundant Cast ops
     # should be needed here.
 
-    input_shape = input.shape
-    if input_shape is not None:
-        output.shape = input_shape.copy()
+    output.shape = _merge_shapes(output.shape, input.shape)
 
     input_dtype = _get_input_element_type(node, 0)
     output_dtype = _get_int_attribute(node, "to", None)