[Draft] Add paddle as backend

[HydrogenSulfate]

Modulus Pull Request

Description

Checklist

• [x] I am familiar with the Contributing Guidelines.
• [ ] New or existing tests cover these changes.
• [x] The documentation is up to date with these changes.
• [ ] The CHANGELOG.md is up to date with these changes.
• [ ] An issue is linked to this pull request.

Dependencies

15/78 tests failed for 13 ModuleNotFoundError(2 pysdf + 11 modulus.sym.models.layers) and 2 ImportError(modulus.models.layers has been moved to modulus but modulus-sym)

============================= test session starts ==============================
platform linux -- Python 3.9.17, pytest-7.4.1, pluggy-1.3.0
rootdir: /workspace/modulus-sym_paddle_backend/test
plugins: hydra-core-1.3.2, anyio-3.7.1, dash-2.13.0
collected 78 items / 15 errors