Topic Modeling: Change LdaModel to LdaMultiCore

**Is your feature request related to a problem? Please describe.**
LDA can be faster.

**Describe the solution you'd like**
Replace the current LdaModel from gensim with its LdaMultiCore.

**Describe alternatives you've considered**
Write parallelization on our own?

**Additional context**
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Locate the current topic-modeling implementation and its tests, then inspect how the existing gensim LdaModel is configured and invoked. Compare the LdaMultiCore interface and verify that the topic-modeling workflow and its tests still pass with the requested replacement.

Written by the indexing model from the issue text.