[WIP] fix various converters for dynamic shapes

[jaybdub]

• adds shape tracking with wrapper classes

Fixed:

• [x] flatten
• [x] squeeze
• [x] unsqueeze
• [x] view
• [x] reshape
• [x] interpolate
• [x] getitem
• [ ] chunk
• [ ] split

Chunk / split can return variable number of outputs for dynamic shapes. They currently don't work with ONNX in this context. A good default behavior in this instance needs to be defined, but for now can probably be omitted as not supported by dynamic shapes.