A small error that will cause onnx compilation at fp16 precision to fail

https://github.com/facebookresearch/sam2/blob/2b90b9f5ceec907a1c18123530e92e794ad901a4/sam2/modeling/position_encoding.py#L160

Here the gird is set to be at float32 precision. If we try to complie onnx in fp16 precision, this will report an error. We may modify it to:
```
device: Any = self.positional_encoding_gaussian_matrix.device
dtype: Any = self.positional_encoding_gaussian_matrix.dtype
grid = torch.ones((h, w), device=device, dtype=dtype)
```