non-str key in hparam dict causes exception in logging
🐛 Bug
The _flatten_dict function in LightningLoggerBase assumes that dicts will only have string keys. Passing a hparam like {2:3} will break it.
Please reproduce using the BoringModel
https://colab.research.google.com/drive/1qN5oIzPcH18QSZhAMy9N-YDkeDkc4-fy

To Reproduce
Use following BoringModel and post here

Expected behavior
It should cast to str before logging
Environment
Note: Bugs with code are solved faster ! Colab Notebook should be made public !


IDE: Please, use our python bug_report_model.py template.


Colab Notebook: Please copy and paste the output from our environment collection script (or fill out the checklist below manually).


You can get the script and run it with:
wget https://raw.githubusercontent.com/PyTorchLightning/pytorch-lightning/master/tests/collect_env_details.py
# For security purposes, please check the contents of collect_env_details.py before running it.
* CUDA:
	- GPU:
		- Tesla T4
	- available:         True
	- version:           10.1
* Packages:
	- numpy:             1.18.5
	- pyTorch_debug:     False
	- pyTorch_version:   1.6.0+cu101
	- pytorch-lightning: 0.10.0
	- tqdm:              4.41.1
* System:
	- OS:                Linux
	- architecture:
		- 64bit
		- 
	- processor:         x86_64
	- python:            3.6.9
	- version:           #1 SMP Thu Jul 23 08:00:38 PDT 2020
### Additional context

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