[FEATURE]: Basic Function of ColoTensors

[feifeibear]

Describe the feature

I found that ColoTensor lacks some basic functionalities.

• [x] initialized in shard mode from a torch tensor.
• [x] save and load in a distributed mode.
• [ ] support torch.einsum
• [ ] support conv2d
• [x] flexible assign process group for each tensor.
• [ ] inter-convert ColoTensor <-> torch.Tensor