[WIP] Add features to ADVI

[Sayam753]

• Add progress bar support.
• Test progress bar in different OS.
  (Tested on MacOS 10.15.6, Ubuntu 18.04.4 LTS, Windows 8.1 and Google colab environments)
• Add ParameterConvergence criteria to test convergence.
• Add LowRank Approximation.
• Add LowRank ADVI tests.
• Update quickstart notebook.
• Add an example notebook with hierarchical models and ADVI.

[codecov]

Codecov Report

Merging #310 into master will increase coverage by 0.05%.
The diff coverage is 91.17%.

@@            Coverage Diff             @@
##           master     #310      +/-   ##
==========================================
+ Coverage   90.02%   90.08%   +0.05%     
==========================================
  Files          33       34       +1     
  Lines        2447     2511      +64     
==========================================
+ Hits         2203     2262      +59     
- Misses        244      249       +5     

Impacted Files	Coverage Δ
pymc4/distributions/transforms.py	78.26% <ø> (ø)
pymc4/variational/approximations.py	90.16% <85.71%> (-1.05%)	⬇️
pymc4/variational/callbacks.py	96.77% <96.77%> (ø)
pymc4/variational/__init__.py	100.00% <100.00%> (ø)
pymc4/variational/updates.py	66.66% <100.00%> (+3.03%)	⬆️