Fix BasisEmbedding to allow only-1 bitstrings

[mariaschuld]

Fixes issue #1112.

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit .github/CHANGELOG.md with:

• A one-to-two sentence description of the change. You may include a small working example for new features.
• A link back to this PR.
• Your name (or GitHub username) in the contributors section.

[codecov]

Codecov Report

Merging #1114 (d2a5ff4) into master (94cf6d4) will increase coverage by 2.79%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1114      +/-   ##
==========================================
+ Coverage   94.94%   97.74%   +2.79%     
==========================================
  Files         155      155              
  Lines       11753    11753              
==========================================
+ Hits        11159    11488     +329     
+ Misses        594      265     -329     

Impacted Files	Coverage Δ
pennylane/templates/embeddings/basis.py	100.00% <100.00%> (ø)
pennylane/math/tensorbox.py	95.97% <0.00%> (+1.34%)	⬆️
pennylane/beta/devices/default_tensor.py	95.23% <0.00%> (+1.70%)	⬆️
pennylane/interfaces/tf.py	88.75% <0.00%> (+2.50%)	⬆️
pennylane/devices/default_qubit_tf.py	90.66% <0.00%> (+2.66%)	⬆️
pennylane/math/numpy_box.py	98.55% <0.00%> (+2.89%)	⬆️
pennylane/beta/devices/default_tensor_tf.py	87.50% <0.00%> (+3.12%)	⬆️
pennylane/tape/qnode.py	98.93% <0.00%> (+3.91%)	⬆️
pennylane/interfaces/torch.py	93.06% <0.00%> (+3.96%)	⬆️
pennylane/tape/interfaces/torch.py	100.00% <0.00%> (+5.26%)	⬆️
... and 10 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 94cf6d4...d2a5ff4. Read the comment docs.

[josh146]

Thanks @mariaschuld for getting to this so quickly!

Even though this is very trivial, should we add tests for the all zeros and all ones case, just to make sure it is correctly validated? This will also help catch similar bugs in the future.

[mariaschuld]

Good idea, added new test cases...