`ConfigParser` doesn't need `deepcopy`

[Nic-Ma]

Is your feature request related to a problem? Please describe.
In the current implementation, ConfigParser executes deepcopy for every config item when recursively parse the whole config:

MONAI/monai/bundle/config_parser.py

Line 379 in 8e8dd1b

item_conf = deepcopy(config)

But actually, all the parsing logic just read the config item content and generate a new config object:

MONAI/monai/bundle/reference_resolver.py

Line 295 in 8e8dd1b

ret = type(config)()

So I would suggest to remove this deepcopy logic, it can bring 3 benefits:

1. If use python object in the config dict, deepcopy it may cause Invertd failure due to mismatch object ID.
2. If the config structure contains very big python object (like CacheDataset), it will increase memory usage, especially considering it's a recursive operation.
3. Some python objects may not support pickle operation and deepcopy will fail.

[Nic-Ma]

CC @KumoLiu @yiheng-wang-nv

[yiheng-wang-nv]

Update: I modified the link @Nic-Ma posted within the ticket and fixed the link with a commit before related changes. It is helpful for further check.