Fix assignment of str-typed value to device attribute

The `device` property is defined via the method
```
@Property
def device(self) -> torch.device:
    return self._device
```
which indicates that the `_device` attribute should only have the `torch.device` datatype, whereas the current implementation allows assigning a `str` value via the `_init_shape(...)` method, which has a parameter `device` typed as `str | Device`.

tensordict/memmap.py

@@ -302,7 +302,7 @@ def _init_shape(
         dtype: torch.dtype,
         transfer_ownership: bool,
     ):
-        self._device = device
+        self._device = torch.device(device)
         self._shape = shape
         self._shape_indexed = None
         self.np_shape = tuple(self._shape)