No gradient required for q_target calculation

Hi @philtabor,
There is a possible bug in the dqn_agent.py file at line 93:
```
q_target = rewards + self.gamma*q_next
```
needs to be replaced with:
```
with torch.no_grad():
      q_target = rewards + self.gamma*q_next
```
This issue is also raised in https://github.com/philtabor/Deep-Q-Learning-Paper-To-Code/issues/9#issue-723686152

Could you please take a look?
