[webgpu] need to have a generic way to cast int64 to uint32 for dims of shapes

[guschmue]

since webgpu does not do well on int64 we cast dims of shapes from int64 to uint32 in many places.
To avoid lots of random static_cast or onnxruntime::narrow we should have a generic way, ie a method on the Shape class that returns the dims as checked uint32.