ConfigDict does not allow dictionary key to be int type

[Michael-H777]

Describe the bug

ConfigDict() does not allow dictionary with int data type as key

To Reproduce

from ml_collections import ConfigDict

tmp = ConfigDict()
tmp.demo0 = 10                         # no bug 
tmp.demo1 = '10'                       # no bug 
tmp.demo2 = {'0': '10', '1': '20'}     # no bug 
tmp.demo3 = {0: '10', 1: '20'}         # bug 

ConfigDict

ConfigFlags

im not sure how to provide config flags, therefore i created a brand new conda virtual env to demonstrate the bug.

~$ conda create --name bug python=3.8
~$ conda activate bug
~$ pip install ml-collections
~$ python

    from ml_collections import ConfigDict
    tmp = ConfigDict()
    tmp.demo = {0: '10', 1: '20'}

Expected behavior

the should run, and save the dictionary without issue

Environment:

• OS: ubuntu
• OS Version: 20
• Python: 3.8

Additional context

when I changed the key to str type, the code runs. However I want to be able to use str type.