Move towards consistent use of the term Tensor

[alewis]

Right now there's a bit of a confusing situation going on where the term Tensor refers both to the type of a given backend array (the old use) and to tn.Tensor itself. Since this is confusing for new developers I recommend we change the former use to something more descriptive, e.g. "backend array". Using that name (subject to discussion) as a placeholder, I would recommend the following:

1. Change convert_to_tensor to e.g. convert_to_backend_array.
2. The type hint Tensor within the backend files should be changed to e.g. BackendArray. Ideally we'd also do something a bit more specific than just BackendArray = Any, though I understand this might be hard given how our build works.

Those are the important things, I think, but in addition:

3. We might consider changing Node so that it wraps tn.Tensor rather than a backend array directly. (Edit: let's actually not do this)