Deprecated ObjectConf for TargetConf

[omry]

This is a partial implementation of #566.

TLDR

Object configs is changing from:

target: classname
params:
  a: 10
  b: 20

to:

_target_: classname
a: 10
b: 20

This include Hydra's launchers and sweepers, so if they appear in your config file you will need to adjust the configs.

Details

ObjectConf:

@dataclass
class ObjectConf(Dict[str, Any]):
    # class, class method or function name
    target: str = MISSING

    # parameters to pass to cls when calling it
    params: Any = field(default_factory=dict)

TargetConf:

@dataclass
class TargetConf:
    """
    Use by subclassing and adding fields
    e.g:
    @dataclass
    class UserConf(TargetConf):
        _target_ = "module.User"
        name: str = MISSING
        age: int = MISSING
    """

    # class, class method or function name
    _target_: str = MISSING

This will be deprecated for user code, but for Hydra it's a breaking change because sweeper and launchers will no longer need the pesky params to override their parameters:

$ python foo.py hydra.sweeper.params.max_batch_size=2
# will become
$ python foo.py hydra.sweeper.max_batch_size=2

[omry]

Possible alternative designs/names for TargetConf.
Happy to discuss this while it's still hot.

# 1
@dataclass
class TargetConf:
    # class, class method or function name
    _target_: str = MISSING

#2 
@dataclass
class CallConf:
    _callable_: str = MISSING

#3 
@dataclass
class CallConf:
   # one of:
   _class_ : str = MISSING      # class only
   _function_ : str = MISSING   # function only
   _callable_: str = MISSING    # either class or function

[shagunsodhani]

My vote is for 1. It is clean, simpler and unambiguous.