FrozenConfigDict can't initialize from ConfigDict with tuples

[malmaud]

Thanks for open-sourcing this nice package!

I don't think I see why in principle FrozenConfigDict can't include fields which are tuples of frozen config dicts, given that Python tuples themselves are already frozen. Is is just a matter of not adding support yet, or is there something more fundamental?

For example,

FrozenConfigDict({'a': ({'b': 1}, {'b': 2})})

gives the error:

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-133-a093d88df5a2> in <module>
----> 1 c=FrozenConfigDict({'a': ({'b': 1}, {'b': 2})})

~/opt/anaconda3/lib/python3.7/site-packages/ml_collections/config_dict/config_dict.py in __init__(self, initial_dictionary, type_safe)
   1635                                     type_safe=type_safe)
   1636 
-> 1637     _frozenconfigdict_valid_input(initial_configdict)
   1638     # This will define the self._configdict attribute
   1639     _frozenconfigdict_fill_seed(self, initial_configdict)

~/opt/anaconda3/lib/python3.7/site-packages/ml_collections/config_dict/config_dict.py in _frozenconfigdict_valid_input(obj, ancestor_list)
   1418   if isinstance(obj, ConfigDict):
   1419     for value in obj.values():
-> 1420       _frozenconfigdict_valid_input(value, ancestor_list)
   1421   elif isinstance(obj, FieldReference):
   1422     _frozenconfigdict_valid_input(obj.get(), ancestor_list)

~/opt/anaconda3/lib/python3.7/site-packages/ml_collections/config_dict/config_dict.py in _frozenconfigdict_valid_input(obj, ancestor_list)
   1424     for element in obj:
   1425       if isinstance(element, (dict, ConfigDict, FieldReference)):
-> 1426         raise ValueError('Bad FrozenConfigDict initialization: Cannot '
   1427                          'contain a dict, ConfigDict, or FieldReference '
   1428                          'within a list or tuple.')

ValueError: Bad FrozenConfigDict initialization: Cannot contain a dict, ConfigDict, or FieldReference within a list or tuple.

[radkoff]

Support for lists would be great!