Fix is_sampled and its test

[antalszava]

Context:

The is_sampled attribute of a quantum tape is required for generating samples on the device level. The behaviour for this attribute was modified in #1027 and #1040.

Due to a bug, this attribute is not being set currently, although its test case is passing. The reason is that the underlying device gets swapped inside the test case, ultimately leading to unexpected behaviour.

Description of the Change:

• Adds the required line for updating the is_sampled attribute
• Fixes the test case

Benefits:
Sampling works correctly.

Possible Drawbacks:
N/A

Related GitHub Issues:
N/A

[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 #1126 (5736b55) into master (10fde20) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1126   +/-   ##
=======================================
  Coverage   97.74%   97.74%           
=======================================
  Files         155      155           
  Lines       11783    11784    +1     
=======================================
+ Hits        11517    11518    +1     
  Misses        266      266           

Impacted Files	Coverage Δ
pennylane/tape/tapes/tape.py	98.34% <100.00%> (+<0.01%)	⬆️

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[mariaschuld]

Thanks so much for fixing this!