Check off a todo in utils: add 'freeze()' to freeze config.

[JosephCatrambone]

Adds a three-line method which uses namedtuple to create frozen configs if one wants to avoid footguns. Checks off the 'todo' item in config. Elements of the config are still accessible by name and as dicts.

Example usage:

>>> cfg = CfgNode(a=0, b=0)
>>> cfg.merge_from_args(['--a=2', '--b=3'])
command line overwriting config attribute a with 2
command line overwriting config attribute b with 3
>>> cfg_frozen = cfg.freeze()
>>> cfg_frozen.a
2
>>> a_frozen.a = 3
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
Cell In[18], line 1
----> 1 cfg_frozen.cfg = 3                                                                                                                                                                                                                        
                                                                                                                       
AttributeError: can't set attribute