[FEATURE REQUEST] decode string values for yaml configs automatically

[hiyyg]

Currently when the config file is defined in a ".yaml" file, the config values are decoded as strings. However, yacs can automatically decode the string values
(https://github.com/rbgirshick/yacs/blob/master/yacs/config.py#L410).
For example, a value might be defined as a tuple:

A: 
  B: ("b1", "b2") 

but yaml loader will load it as a string.

Is it possible to add this feature into mmcv?