Doc styler examples (#14953)

* Fix bad examples

* Add black formatting to style_doc

* Use first nonempty line

* Put it at the right place

* Don't add spaces to empty lines

* Better templates

* Deal with triple quotes in docstrings

* Result of style_doc

* Enable mdx treatment and fix code examples in MDXs

* Result of doc styler on doc source files

* Last fixes

* Break copy from

docs/source/benchmarks.mdx

@@ -37,11 +37,12 @@ The benchmark classes [`PyTorchBenchmark`] and [`TensorFlowBenchmark`] expect an
 
 >>> args = PyTorchBenchmarkArguments(models=["bert-base-uncased"], batch_sizes=[8], sequence_lengths=[8, 32, 128, 512])
 >>> benchmark = PyTorchBenchmark(args)
-
 ===PT-TF-SPLIT===
 >>> from transformers import TensorFlowBenchmark, TensorFlowBenchmarkArguments
 
->>> args = TensorFlowBenchmarkArguments(models=["bert-base-uncased"], batch_sizes=[8], sequence_lengths=[8, 32, 128, 512])
+>>> args = TensorFlowBenchmarkArguments(
+...     models=["bert-base-uncased"], batch_sizes=[8], sequence_lengths=[8, 32, 128, 512]
+... )
 >>> benchmark = TensorFlowBenchmark(args)
 ```
 
@@ -174,7 +175,9 @@ configurations must be inserted with the benchmark args as follows.
 ```py
 >>> from transformers import PyTorchBenchmark, PyTorchBenchmarkArguments, BertConfig
 
->>> args = PyTorchBenchmarkArguments(models=["bert-base", "bert-384-hid", "bert-6-lay"], batch_sizes=[8], sequence_lengths=[8, 32, 128, 512])
+>>> args = PyTorchBenchmarkArguments(
+...     models=["bert-base", "bert-384-hid", "bert-6-lay"], batch_sizes=[8], sequence_lengths=[8, 32, 128, 512]
+... )
 >>> config_base = BertConfig()
 >>> config_384_hid = BertConfig(hidden_size=384)
 >>> config_6_lay = BertConfig(num_hidden_layers=6)
@@ -244,7 +247,9 @@ bert-6-lay                 8              512            1359
 ===PT-TF-SPLIT===
 >>> from transformers import TensorFlowBenchmark, TensorFlowBenchmarkArguments, BertConfig
 
->>> args = TensorFlowBenchmarkArguments(models=["bert-base", "bert-384-hid", "bert-6-lay"], batch_sizes=[8], sequence_lengths=[8, 32, 128, 512])
+>>> args = TensorFlowBenchmarkArguments(
+...     models=["bert-base", "bert-384-hid", "bert-6-lay"], batch_sizes=[8], sequence_lengths=[8, 32, 128, 512]
+... )
 >>> config_base = BertConfig()
 >>> config_384_hid = BertConfig(hidden_size=384)
 >>> config_6_lay = BertConfig(num_hidden_layers=6)