Fix posterior method in `BatchedMultiOutputGPyTorchModel` for tracing JIT

[SaiAakash]

Motivation

Fixes #2591. Generates the MTMVN for the independent task case slightly differently when jit traced.

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

A unit test test_posterior_in_trace_mode has been added to test_gpytorch.py

Related PRs

NA