[feat] Allow num_tasks to be None

[vkarak]

Replaces #2768.

As in the discussion in #2693, we now allow num_tasks be None. If set so, it is the user's responsibility to generate a valid job file. Scheduler backends will try not to (re)set num_tasks if it is not specified. See documentation for the exact behaviour of every backend.

[codecov-commenter]

Codecov Report

Base: 86.58% // Head: 86.58% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (7e1e808) compared to base (6c3d163).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #2778   +/-   ##
========================================
  Coverage    86.58%   86.58%           
========================================
  Files           60       60           
  Lines        11291    11293    +2     
========================================
+ Hits          9776     9778    +2     
  Misses        1515     1515           

Impacted Files	Coverage Δ
reframe/core/pipeline.py	93.77% <100.00%> (ø)
reframe/core/schedulers/__init__.py	98.39% <100.00%> (ø)
reframe/core/schedulers/lsf.py	49.36% <100.00%> (+1.26%)	⬆️
reframe/core/schedulers/oar.py	39.56% <100.00%> (+0.67%)	⬆️
reframe/core/schedulers/pbs.py	46.05% <100.00%> (+0.35%)	⬆️
reframe/core/schedulers/slurm.py	52.80% <100.00%> (ø)
reframe/frontend/executors/policies.py	92.54% <0.00%> (-0.28%)	⬇️

Help us with your feedback. Take ten seconds to tell us how you rate us. Have a feature suggestion? Share it here.

☔ View full report at Codecov.
📢 Do you have feedback about the report comment? Let us know in this issue.