replace LambdaLR scheduler wrappers by function

Custom schedulers are currently initiated by wrapping Pytorch's LambdaLR
class and passing a method of the wrapping class to the __init__
function of LambdaLR. This approach is not appropriate for several
reasons:

1. one does not need to define a class when it only defines a
__init__() method;
2. instantiating the parent class by passing a method of the child class
creates a cyclical reference which leads to memory leaks. See issues #1742 and #1134.

In this commit we replace the wrapper classes with functions that
instantiate `LambdaLR` with a custom learning rate function. We use a
closure to specify the parameter of the latter. We also do a bit of
renaming within the function to explicit the behaviour and removed
docstrings that were subsequently not necessary.

transformers/__init__.py

@@ -97,8 +97,8 @@ if is_torch_available():
     from .modeling_encoder_decoder import PreTrainedEncoderDecoder, Model2Model
 
     # Optimization
-    from .optimization import (AdamW, ConstantLRSchedule, WarmupConstantSchedule, WarmupCosineSchedule,
-                               WarmupCosineWithHardRestartsSchedule, WarmupLinearSchedule)
+    from .optimization import (AdamW, get_constant_schedule, get_constant_schedule_with_warmup, get_cosine_schedule_with_warmup,
+                               get_cosine_with_hard_restarts_schedule_with_warmup, get_linear_schedule_with_warmup)
 
 
 # TensorFlow