Compatibility fix with Hydra 1.1 (#3722)

Summary:
One of the changes in Hydra 1.1 is that the default composition order is changing.
This is documented [here](https://hydra.cc/docs/upgrades/1.0_to_1.1/default_composition_order).
In Hydra 1.1, a config is overriding values introduced by the defaults list while in Hydra 1.0 - the defaults list was overriding the values in the config.

fairseq is currently depending on the previous behavior:
The class `FairseqConfig` defines config values, and it's expecting them to be overridden by the defaults list.
This result in a different config being created when running `fairseq_cli/hydra_train.py` with Hydra 1.0 and with 1.1.

Hydra 1.1 introduced the `_self_` keyword in the defaults list to control the composition order. In order to achieve the behavior of Hydra 1.0, `_self_` should be added as the first item in the defaults list.

To allow for a smoother migration, Hydra 1.0 is ignoring `_self_` starting from 1.0.7 (previous versions will issue an error).

This diff adds `_self_` as the first item in the defaults list the fairseq config, and introduce a dependency a Hydra 1.0 version that is equal or newer to 1.0.7.

### Testing:
I ensured that the following yield the same composed config:
Default config with Hydra 1.0.6, 1.0.7 and 1.1.0

`examples/wav2vec/config/finetuning/base_10h.yaml` with Hydra 1.0.6, 1.0.7 and 1.1.0.

This can be achieved by outputing the generated config using `--cfg job` and compating the outputs.

Pull Request resolved: https://github.com/pytorch/fairseq/pull/3722

Reviewed By: dianaml0

Differential Revision: D29917677

Pulled By: jieru-hu

fbshipit-source-id: 7e645b83cccb03fc80a6702e302c4643d2b14a78

fairseq/config/config.yaml

@@ -5,6 +5,7 @@ hydra:
     dir: .
 
 defaults:
+    - _self_
     - task: null
     - model: null
     - criterion: cross_entropy

fairseq/optim/fp16_optimizer.py

@@ -479,7 +479,7 @@ class MemoryEfficientFP16Optimizer(
                 "Unsupported optimizer: {}".format(optimizer.__class__.__name__)
             )
 
-        super().__init__(cfg.optimizer)
+        super().__init__(getattr(cfg, "optimizer", None))
         self.wrapped_optimizer = optimizer
 
         if getattr(cfg.common, "fp16_scale_window", None) is None:

setup.py

@@ -201,7 +201,7 @@ def do_setup(package_data):
             "cffi",
             "cython",
             'dataclasses; python_version<"3.7"',
-            "hydra-core<1.1",
+            "hydra-core>=1.0.7,<1.1",
             "omegaconf<2.1",
             'numpy<1.20.0; python_version<"3.7"',
             'numpy; python_version>="3.7"',