[Encoding] Refresh practical encodings to follow the naming convention.

[hanhanW]

• iree_encoding.pad_encoding_layout -> iree_encoding.padding
• iree_gpu.gpu_pad_layout -> iree_gpu.gpu_padding_resolver
• iree_cpu.cpu_encoding_layout -> iree_cpu.cpu_encoding_resolver
• iree_cpu.vmvx_encoding_layout -> iree_cpu.vmvx_encoding_resolver
• iree_gpu.gpu_encoding_layout -> iree_gpu.gpu_encoding_resolver

It is a step towards https://github.com/iree-org/iree/issues/20742

[hanhanW]

It based on https://github.com/iree-org/iree/pull/21144

[benvanik]

(thanks as always @hanhanW for continually improving/refining this code <3)