Updates the state learning tutorial for TensorFlow 2.0

[josh146]

Context: The existing TensorFlow tutorial was out of date (requiring the old TF1.3 backend), and in the form of a Jupyter notebook.

Description of the Change:

• Ports the tutorial to Sphinx-Gallery, by using the converter script available here.

• Ports the tutorial to use the new TF2 backend

Benefits: As the tutorial has been ported to Sphinx gallery, it is now executable, making it easy to test to see whether it provides the correct output.

Possible Drawbacks: Had to reduce the number of resources and optimization steps, since this tutorial will now be executed alongside the docs.

Related GitHub Issues: n/a

[codecov[bot]]

Codecov Report

Merging #373 into tf2 will decrease coverage by 31.53%. The diff coverage is n/a.

@@             Coverage Diff             @@
##              tf2     #373       +/-   ##
===========================================
- Coverage   97.76%   66.22%   -31.54%     
===========================================
  Files          52       52               
  Lines        6387     6387               
===========================================
- Hits         6244     4230     -2014     
- Misses        143     2157     +2014     

Impacted Files	Coverage Δ
strawberryfields/apps/subgraph.py	10.11% <0.00%> (-89.89%)	:arrow_down:
strawberryfields/apps/clique.py	10.67% <0.00%> (-89.33%)	:arrow_down:
strawberryfields/apps/similarity.py	14.87% <0.00%> (-85.13%)	:arrow_down:
strawberryfields/apps/sample.py	15.78% <0.00%> (-84.22%)	:arrow_down:
strawberryfields/io.py	14.63% <0.00%> (-84.15%)	:arrow_down:
strawberryfields/decompositions.py	20.00% <0.00%> (-79.60%)	:arrow_down:
strawberryfields/backends/gaussianbackend/ops.py	20.27% <0.00%> (-77.03%)	:arrow_down:
strawberryfields/configuration.py	24.59% <0.00%> (-75.41%)	:arrow_down:
...fields/backends/gaussianbackend/gaussiancircuit.py	24.04% <0.00%> (-74.43%)	:arrow_down:
strawberryfields/cli/__init__.py	19.17% <0.00%> (-71.24%)	:arrow_down:
... and 25 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 89ff81b...c431f70. Read the comment docs.

[josh146]

Rendered screenshot here

[josh146]

I'm also worried if this will finally stop us being able to build on RTD 🤔 Not because this tutorial is particularly long (it took me 2 mins locally), but we just seem close!

Let's see how it goes 😆 I've got my fingers crossed, because we're not close to hitting the timeout on read the docs (instead I'm more worried about memory usage). I'm beginning work on the new photonics-applications repo, so we should be able to move the tutorials very soon.