Unite Session from qiskit_ibm_provider with Session in this repository

[merav-aharoni]

Summary

IBMBackend.run() now uses the same Session as used by the primitives.

Details and comments

I made changes in the imports because of cyclic imports.
IBMBackend can receive a Session in 3 ways:

1. As a context manager (as for primitives)
2. As a parameter to IBMBackend.run(). This makes more sense than setting it in __init__, because often the user will define the backend as backend = service.get_backend(), so the backend will keep the same Session object even if the session is closed, causing failures. Even if we call backend = service.get_backend() again, backend will still be the same object as before.
3. Using IBMBackend.open_session(). I believe this should eventually be deprecated.

[coveralls]

Pull Request Test Coverage Report for Build 7749509110

• -5 of 17 (70.59%) changed or added relevant lines in 3 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.001%) to 69.15%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
qiskit_ibm_runtime/ibm_backend.py	6	11	54.55%

Totals
Change from base Build 7747929718:	0.001%
Covered Lines:	5485
Relevant Lines:	7932

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💛 - Coveralls

[kt474]

With the upcoming deprecation of backend.run() I will close this for now