Setting of PYTHONHASHSEED has no effect
🐛 Bug
(Previously submitted here: #1939, but I didn't use the correct template, so now I'm resubmitting)
In 
  
    
      pytorch-lightning/pytorch_lightning/trainer/seed.py
    
    
         Line 32
      in
      9045b6c
    
  
  
    

        
          
           os.environ["PYTHONHASHSEED"] = str(seed) 
        
    
  


, PYTHONHASHSEED is assigned a value in order to ensure reproducability. However, this assignment has no effect. In fact, this assignment might mislead the user or any logging software into believing that PYTHONHASHSEED has a specific value, when in fact it has another.
To see that setting PYTHONHASHSEED inside the current program has no effect, run the following two commands:
PYTHONHASHSEED=1 python -c "import os; print(hash('a'))"
PYTHONHASHSEED=1 python -c "import os; os.environ['PYTHONHASHSEED']='2'; print(hash('a'))"

The commands should output the same value, meaning that setting PYTHONHASHSEED after the process has started has no effect.
The following commands will likely output different values, also indicating that setting PYTHONHASHSEED after the process has started has no effect:
unset PYTHONHASHSEED # make sure it is not already set
python -c "import os; os.environ['PYTHONHASHSEED']='2'; print(hash('a'))"
python -c "import os; os.environ['PYTHONHASHSEED']='2'; print(hash('a'))"


To Reproduce
Steps to reproduce the behavior:

Start python terminal with PYTHONHASHSEED=1 python
Run

import pytorch_lightning as pl
pl.seed_everything(100)
print(hash('a'))
# >>> 8432517439229126278

Start new python terminal with PYTHONHASHSEED=2 python
Run

import pytorch_lightning as pl
pl.seed_everything(100)
print(hash('a'))
# >>> -8333094867672744108
Expected behavior
Expect output of hash function to be the same in both cases. The examples demonstrate that this is not possible.

Environment
* CUDA:
	- GPU:
	- available:         False
	- version:           10.2
* Packages:
	- numpy:             1.18.5
	- pyTorch_debug:     False
	- pyTorch_version:   1.5.0
	- pytorch-lightning: 0.7.6
	- tensorboard:       2.2.2
	- tqdm:              4.46.1
* System:
	- OS:                Linux
	- architecture:
		- 64bit
		- ELF
	- processor:         
	- python:            3.8.3
	- version:           #1 SMP PREEMPT Wed May 27 20:25:12 UTC 2020