Docs build broken with Sphinx v3.1.1

[matthewfeickert]

Description

After the new Sphinx patch release v3.1.1 was released there is an error with building the docs due to autodocumenting:

WARNING: don't know which module to import for autodocumenting 'optimize.opt_jax.jax_optimizer' (try placing a "module" or "currentmodule" directive in the document, or giving an explicit module name)
WARNING: don't know which module to import for autodocumenting 'optimize.opt_minuit.minuit_optimizer' (try placing a "module" or "currentmodule" directive in the document, or giving an explicit module name)
WARNING: don't know which module to import for autodocumenting 'optimize.opt_pytorch.pytorch_optimizer' (try placing a "module" or "currentmodule" directive in the document, or giving an explicit module name)
WARNING: don't know which module to import for autodocumenting 'optimize.opt_scipy.scipy_optimizer' (try placing a "module" or "currentmodule" directive in the document, or giving an explicit module name)
WARNING: don't know which module to import for autodocumenting 'optimize.opt_tflow.tflow_optimizer' (try placing a "module" or "currentmodule" directive in the document, or giving an explicit module name)
WARNING: don't know which module to import for autodocumenting 'tensor.jax_backend.jax_backend' (try placing a "module" or "currentmodule" directive in the document, or giving an explicit module name)
WARNING: don't know which module to import for autodocumenting 'tensor.numpy_backend.numpy_backend' (try placing a "module" or "currentmodule" directive in the document, or giving an explicit module name)
WARNING: don't know which module to import for autodocumenting 'tensor.pytorch_backend.pytorch_backend' (try placing a "module" or "currentmodule" directive in the document, or giving an explicit module name)
WARNING: don't know which module to import for autodocumenting 'tensor.tensorflow_backend.tensorflow_backend' (try placing a "module" or "currentmodule" directive in the document, or giving an explicit module name)

[matthewfeickert]

This is escalated to Sphinx Issue 7844.