Added model for MPIJob

[yindia]

TL;DR

• Added MPIJob in the task model. it is required for mpi plugins Added mpi plugin #595

Type

• Bug Fix
• Feature
• Plugin

Are all requirements met?

• Code completed
• Smoke tested
• Unit tests added
• Code documentation added
• Any pending items have an associated Issue

Complete description

How did you fix the bug, make the feature etc. Link to any design docs etc

Tracking Issue

flyteorg/flyte#1543

Follow-up issue

NA
OR
https://github.com/lyft/flyte/issues/

[codecov]

Codecov Report

Merging #709 (7a7938a) into master (eeb66b3) will decrease coverage by 0.01%.
The diff coverage is 60.00%.

❗ Current head 7a7938a differs from pull request most recent head 321ff5d. Consider uploading reports for the commit 321ff5d to get more accurate results

@@            Coverage Diff             @@
##           master     #709      +/-   ##
==========================================
- Coverage   85.72%   85.71%   -0.02%     
==========================================
  Files         357      357              
  Lines       29729    29749      +20     
  Branches     2428     2428              
==========================================
+ Hits        25486    25498      +12     
- Misses       3603     3611       +8     
  Partials      640      640              

Impacted Files	Coverage Δ
flytekit/models/task.py	90.75% <60.00%> (-1.39%)	⬇️

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[kumare3]

you do not need to add it here, it can be in the plugin repo itself