Ani 2x (#480)

* Adding ANI-2x parameters

* Moving 2x to seperate branch

* Adding ANI-2x to new branch

* Adding 2x network

* Update models.py

* Update api.rst

* Update __init__.py
Co-authored-by: Christian Devereux <cdever01@ufl.edu>
Co-authored-by: Farhad Ramezanghorbani <farhadrgh@users.noreply.github.com>

torchani/resources/ani-2x_8x/train6/networks/ANN-F.nnf-l3.bparam

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torchani/resources/ani-2x_8x/train6/networks/ANN-F.nnf-l4.bparam

+8
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torchani/resources/ani-2x_8x/train6/networks/ANN-H.nnf-l3.bparam

--broken encoding: IBM420_ltr

torchani/resources/ani-2x_8x/train6/networks/ANN-H.nnf-l4.bparam

+A
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