Test `MemmapTensor` creation with shape, dtype and device

[vmoens]

MemmapTensors can be created via MemmapTensor(tensor), MemmapTensor(memmap_tensor), MemmapTensor(array) and MemmapTensor(*size, device=device, dtype=dtype).
We don't explicitely test those four methods of creating a memmap tensor.

The test should include:

• tests regarding device: is the device the one expected (using get_available_devices() and pytest.mark.parameterize). Is the device accurate after calling to(device)?
• similar tests for dtype
• testing that inputting a zero-tensor / zero-array / zero-memmap to MemmapTensor results in the same tensor (ie has the same value) than creating a MemmapTensor via its shape

[fredfung007]

MemmapTensor(array) is not supported for now:

https://github.com/facebookresearch/rl/blob/dbc7ba0293d2d3321f4285ae2f21ee2c7d5c6dcd/torchrl/data/tensordict/memmap.py#L77-L79

[vmoens]

Let's leave arrays aside for now :)