Fix: remove device_map from SmolVLA model loading (#3029)

* Fix SmolVLA meta tensor error by removing device_map

- Remove device_map parameter from VLM model loading
- Change torch_dtype from string to torch.bfloat16
- Add explicit .to(device) calls after initialization

This resolves NotImplementedError when training SmolVLA policy.
Fixes meta tensor copy issue in factory.py:418.

* fix: remove manual device movement logic and fix dtype handling

---------

Co-authored-by: Highsky7 <albert31115@gmail.com>

src/lerobot/policies/smolvla/smolvlm_with_expert.py

@@ -77,7 +77,6 @@ class SmolVLMWithExpertModel(nn.Module):
             print(f"Loading  {model_id} weights ...")
             self.vlm = AutoModelForImageTextToText.from_pretrained(
                 model_id,
-                device_map=device,
                 torch_dtype="bfloat16",
                 low_cpu_mem_usage=True,
             )