Surgeon ops do not support networks with dynamic input size

[gchlebus]

Would it be possible to support networks, which have dynamic input shapes (e.g., Input(shape=(channels, None, None)) for 2D networks)? Currently, an exception is being thrown by this line: https://github.com/BenWhetton/keras-surgeon/blob/459e8dfcd9606a35299468a5e9c18b27453d3cfb/src/kerassurgeon/surgeon.py#L244