`fit_gyptorch_model` refactor (#1371)

[j-wilson]

Summary: X-link: https://github.com/pytorch/botorch/pull/1371

This commit updates fit_gpytorch_model and related methods, with the aim of fixing existing issues and improving extensibility. Key changes are as follow:

• Replace fit_gpytorch_model with fit_gpytorch_mll, a Dispatcher backed reimplementation of the original model fitting pipeline. Note that fit_gpytorch_mll does not pass kwargs to optimizer and instead introduces an optional optimizer_kwargs argument.

• Convert fit_gpytorch_model into a convenience method for calling fit_gpytorch_mll with (limited) support for legacy API.

• Add validation for multioutput GP fitting routines based on decomposing a single model into a list of independent models.

• Updated unit tests for relevant code paths.

Differential Revision: D38692173

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D38692173

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D38692173

[facebook-github-bot]

This pull request was exported from Phabricator. Differential Revision: D38692173