[MRG] Doc2Vec inference, notebook cleanup

[gojomo]

Inference improvements: default parameters to analogous model parameters

Notebook updates: test operation, correct errors, provide updated executions

[gojomo]

infer_vector() now defaults to using the analogous alpha/min_alpha/epochs from model initialization unless alternate values specified. Old steps name for epochs still supported for backward-compatibility.

doc2vec-lee slight changes. No longer uses odd 55 epochs - the #2061 fix made the inference-self-checks more stable with fewer passes.

doc2vec-IMDB full executability restored & verified. Errors in leading text corrected, other explanations tweaked. ~2h text preprocessing introduced by naive bash-to-python translation now optimized to ~1m. Fancy optimizations & parallel-model-operations that could confuse beginners removed. Re-inference evaluations moved to optional bottom.

[piskvorky]

@gojomo can you fix the CI errors, so we can merge?

[gojomo]

As noted in discussion elsewhere, the CI issues were unrelated to any changes in this PR.