fix: docvec equality if tensors are involved

[JohannesMessner]

Before this, DocVec equality checks that involved a tensor would fail by raising and Exception:

    class Text(BaseDoc):
        tens: TorchTensor

    da = DocVec[Text](
        [Text(tens=[1, 2, 3, 4]) for _ in range(10)], tensor_type=TorchTensor
    )
    da2 = DocVec[Text](
        [Text(tens=[1, 2, 3, 4]) for _ in range(10)], tensor_type=TorchTensor
    )
    assert da == da2  # would raise

RuntimeError: Boolean value of Tensor with more than one value is ambiguous

This is because checks of the form tens1 == tens2 return a boolean tensor, not a single bool, which breaks an assumption in the previous implementation.

This PR solved that by introducing a helper function that let's us compare tensors and get a single bool as output.

[github-actions]

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