Update tutorial examples to thread explicit RNGs

[samanklesaria]

Closes #4971

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[samanklesaria]

When using Jupyter to edit the notebooks, I find that all the {code-cell} blocks in the markdown get replaced with {code-cell} ipython3. That is, jupytext on my machine labels all code cells with the language, while on the main branch, code cells are inconsistently labeled. In the mnist tutorial there's no ipython3 label, but in the randomness tutorial there is. Instead of figuring out how to configure my jupyter settings separately for each notebook file, I'm just going to leave this commit including the extra ipython3 labels.

[cgarciae]

Thanks @samanklesaria ! Its looking great. Left some comments.
Maybe the other place we can update is the index.rst which also contains an example that uses dropout.