Learners Isolation

[AymenFJA]

An issue was discovered when scaling ROSE as a Service.

For learners that run executable tasks and produce on-disk files to be processed by other tasks within the same Learner, collisions occur due to similar file names.

The fix is to isolate the Learners in a separate sandbox to avoid such an issue.

The directory will be created upon learner initialization by:

1. Users specify their desired cwd for the learner/learners.
2. ROSE automatically on the startup of each Learner inject and create a dir for each Learner

In both cases, the specified cwd will be propagated as follows through our stack: ROSE-->AsyncFlow-->RHAPSODY.