set minimal req. PT 1.4

[Borda]

What does this PR do?

for lack of older PT version on Conda we drop 1.3 from testing

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[Borda]

some strange error, but unreleted:

platform linux -- Python 3.6.12, pytest-5.0.0, py-1.10.0, pluggy-0.13.1 -- /opt/hostedtoolcache/Python/3.6.12/x64/bin/python
cachedir: .pytest_cache
rootdir: /home/runner/work/pytorch-lightning/pytorch-lightning, inifile: setup.cfg
plugins: hydra-core-1.0.0
Fatal Python error: Segmentation fault

[codecov]

Codecov Report

Merging #5418 (26eb369) into release/1.2-dev (51b9df3) will increase coverage by 0%.
The diff coverage is n/a.

@@               Coverage Diff               @@
##           release/1.2-dev   #5418   +/-   ##
===============================================
  Coverage               93%     93%           
===============================================
  Files                  151     151           
  Lines                10617   10617           
===============================================
+ Hits                  9829    9835    +6     
+ Misses                 788     782    -6