[converter] support new PyTorch operators

Below are the PyTorch operators that are yet to be supported.

## Unclassfied (New)
N/A

## Primitives (Python operators)
- [x] aten::len

## Very easy (Constant generation or aliasing)
- [x] aten::clamp_min
- [x] aten::clamp_max
- [x] aten::expand_as

## Easy (Direct mapping)

## Medium (Composite of multiple operators)
- [x] aten::im2col https://github.com/alibaba/TinyNeuralNetwork/issues/69
- [x] aten::col2im https://github.com/alibaba/TinyNeuralNetwork/issues/69
- [x] aten::mish
- [x] aten::group_norm
- [ ] torchvision::nms https://github.com/alibaba/TinyNeuralNetwork/issues/16

## Hard (No mapping or the mapping is too complex)
- [ ] aten::grid_sample https://github.com/alibaba/TinyNeuralNetwork/issues/69
- [ ] quantized::instance_norm
- [ ] quantized::layer_norm

The issue lists unsupported PyTorch operators, grouped by implementation difficulty, but names no source files, tests, or entry points. Start by reviewing the unchecked operators and the linked issues 16 and 69; completion would require an agreed mapping and verification for the selected operator or operators.

Written by the indexing model from the issue text.