Time slice cross validation MMM Notebook (pymc-labs#971)

* nb init

* add param stability

* crps init

* add crps to package

* make mypy happy

* clean

* relax req

* update path

* add crps test

* first iteration

* improvements

* improve tests

* improve tests

* improve references

* feedback 1

* add examples

Author: juanitorduz

docs/source/api/index.md

@@ -13,5 +13,6 @@
   model_config
   model_builder
   prior
+  metrics
   mlflow
 ```

docs/source/notebooks/index.md

@@ -13,6 +13,7 @@ mmm/mmm_tvp_example
 mmm/mmm_time_varying_media_example
 mmm/mmm_components
 mmm/mmm_roas
+mmm/mmm_time_slice_cross_validation
 :::
 
 :::{toctree}