Restructure and improve docs in apps.train module

[trbromley]

This PR does the following:

• Changes the import style so that key classes are available directly within apps.train
• Collates the embed, param, and cost module docstrings into the apps.train docstring

[codecov[bot]]

Codecov Report

Merging #414 into master will increase coverage by 0.04%. The diff coverage is 99.17%.

@@            Coverage Diff             @@
##           master     #414      +/-   ##
==========================================
+ Coverage   97.84%   97.89%   +0.04%     
==========================================
  Files          58       67       +9     
  Lines        6784     6933     +149     
==========================================
+ Hits         6638     6787     +149     
  Misses        146      146              

Impacted Files	Coverage Δ
strawberryfields/api/result.py	100.00% <ø> (ø)
strawberryfields/apps/train/embed.py	100.00% <ø> (ø)
...awberryfields/backends/gaussianbackend/__init__.py	100.00% <ø> (ø)
strawberryfields/backends/gaussianbackend/ops.py	100.00% <ø> (+2.70%)	:arrow_up:
strawberryfields/circuitspecs/gaussian_unitary.py	100.00% <ø> (ø)
strawberryfields/circuitspecs/tensorflow.py	100.00% <ø> (ø)
strawberryfields/backends/fockbackend/ops.py	94.04% <94.11%> (+2.04%)	:arrow_up:
strawberryfields/ops.py	98.84% <96.29%> (-0.19%)	:arrow_down:
strawberryfields/backends/tfbackend/ops.py	96.69% <97.88%> (-0.17%)	:arrow_down:
strawberryfields/apps/data/feature.py	98.36% <98.36%> (ø)
... and 39 more

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