fix slow import: only import quimb.tensor when needed

[anurudhp]

partially addresses #947: reduces import qualtran time from ~3.2s to ~1.4s

[anurudhp]

@mpharrigan ptal

[mpharrigan]

is it accurate to say that the change is only strictly required in controlled.py, since all the other locations aren't actually pulled in when you import qualtran.?

[anurudhp]

is it accurate to say that the change is only strictly required in controlled.py, since all the other locations aren't actually pulled in when you import qualtran.?

Yep. The remaining changes speed up import qualtran.bloqs.basic_gates by ~2s.