[Feature request] Add pretty-print to __repr__

[fmassa]

I think it would be nice to have __repr__ of CfgNode have a prettier print, which has one field per line with proper indentation. It currently prints everything in a single line (as is the default for dict), and makes searching for specific arguments a bit difficult.
While it's possible to use external libraries like pprint to achieve this, I think that having it built-in would be valuable.

A possible draft implementation:

def _addindent(s_, numSpaces):
    s = s_.split("\n")
    # don't do anything for single-line stuff
    if len(s) == 1:
        return s_
    first = s.pop(0)
    s = [(numSpaces * " ") + line for line in s]
    s = "\n".join(s)
    s = first + "\n" + s
    return s

class CfgNode(dict):
   ...
    def __repr__(self):
        r = object.__repr__(self)
        attrs = {k: v for k, v in self.items()}
        if attrs:
            r += ": \n"
            s = []
            for k, v in attrs.items():
                attr_str = "{}: {}".format(repr(k), repr(v))
                attr_str = _addindent(attr_str, 2)
                s.append(attr_str)
            r += "  " + "\n  ".join(s)
        return r

[rbgirshick]

This seems like a reasonable request. I'll implement it.