better reporting of errors when using TP

[sayakpaul]

I think the following aren't implemented at the moment (which is fine; just flagging).

• Sharded loading. from_pretrained should stream shards straight to each rank's DTensor rather than materializing the full checkpoint then slicing — otherwise TP saves you nothing at load time. And check save_pretrained / state_dict calls .full_tensor() or uses DCP. I think we should at least raise when save_pretrained() is called in case TP is enabled?
• LoRA loading. for a colwise base layer, lora_A replicated + lora_B colwise; for rowwise, lora_A rowwise + lora_B replicated. If the plan doesn't cover PEFT layers, loading an adapter onto a TP model will either error or be wrong. I think we should detect if the model has peft layers injected and raise if TP is requested?
• Quantization, offloading. We should probably also raise when these are requested?

Originally posted by @sayakpaul in #13718 (comment)