Update weibull_aft notebook (#4020)

* 1. Fixed: AttributeError: module 'statsmodels' has no attribute 'datasets'
2. Fixed: FutureWarnings
3. Updated the notebook to follow the consistent style

using arviz functions directly to plot trace and summary

1. Fixed: AttributeError: module 'statsmodels' has no attribute 'datasets'
2. Fixed: FutureWarnings
3. Updated the notebook to follow the consistent style

using arviz functions directly

* 1. directly get inference object from sample rather than using from_pymc3 2. Other minor formatting

* remove the 'tune' and 'draws' parameters from sample function call as they were using specifying the default values.

Author: AmitKus