Fix premature session closure error

[takh04]

This issue shows pennylane-qiskit session closes prematurely. Simply adding _if self.session is None: fixes the problem. The fix is tested on both IBM fake device and real devices.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (40a4d24) to head (b15c90e).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master      #608   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            9         9           
  Lines          868       869    +1     
=========================================
+ Hits           868       869    +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[austingmhuang]

Hi @takh04, thanks for opening this PR. The CI is failing right now because of some codefactor issue, likely because we updated our standards and this code hasn't been updated in a while. We can fix this just by appending too-many-positional-arguments to this statement # pylint: disable=too-many-instance-attributes,attribute-defined-outside-init at the top of qiskit_device.py (Line 18).

Additionally, please write a changelog entry in CHANGELOG.md in the section Bug Fixes, and add your name to the list of contributors :)

[takh04]

Hi @austingmhuang, thanks for the reply! I have appended too-many-positional-arguments and added changelog entry!