🚨🚨🚨 Enforce single model initialization (#21431)

* Enforce single model initialization

* Add OneFormer example for problem 3

* Do it the Stas way

* Actually rename the uses...

* Rewrite test

* Try to change the test this way

* Fix all init slow/fast tests

* Break connection

* Fix more tests

* Fix test for initialization

* Remove custom test

* Quality

* Fix last failing tests

* The end?

docs/source/en/add_new_model.mdx

@@ -492,6 +492,48 @@ model = BrandNewBertModel(BrandNewBertConfig())
 The above command will create a model according to the default parameters as defined in `BrandNewBertConfig()` with
 random weights, thus making sure that the `init()` methods of all components works.
 
+Note that all random initialization should happen in the `_init_weights` method of your `BrandnewBertPreTrainedModel`
+class. It should initialize all leaf modules depending on the variables of the config. Here is an example with the
+BERT `_init_weights` method:
+
+```py
+def _init_weights(self, module):
+    """Initialize the weights"""
+    if isinstance(module, nn.Linear):
+        module.weight.data.normal_(mean=0.0, std=self.config.initializer_range)
+        if module.bias is not None:
+            module.bias.data.zero_()
+    elif isinstance(module, nn.Embedding):
+        module.weight.data.normal_(mean=0.0, std=self.config.initializer_range)
+        if module.padding_idx is not None:
+            module.weight.data[module.padding_idx].zero_()
+    elif isinstance(module, nn.LayerNorm):
+        module.bias.data.zero_()
+        module.weight.data.fill_(1.0)
+```
+
+You can have some more custom schemes if you need a special initialization for some modules. For instance, in
+`Wav2Vec2ForPreTraining`, the last two linear layers need to have the initialization of the regular PyTorch `nn.Linear`
+but all the other ones should use an initialization as above. This is coded like this:
+
+```py
+def _init_weights(self, module):
+    """Initialize the weights"""
+    if isinstnace(module, Wav2Vec2ForPreTraining):
+        module.project_hid.reset_parameters()
+        module.project_q.reset_parameters()
+        module.project_hid._is_hf_initialized = True
+        module.project_q._is_hf_initialized = True
+    elif isinstance(module, nn.Linear):
+        module.weight.data.normal_(mean=0.0, std=self.config.initializer_range)
+        if module.bias is not None:
+            module.bias.data.zero_()
+```
+
+The `_is_hf_initialized` flag is internally used to make sure we only initialize a submodule once. By setting it to
+`True` for `module.project_q` and `module.project_hid`, we make sure the custom initialization we did is not overridden later on,
+the `_init_weights` function won't be applied to them.
+
 **6. Write a conversion script**
 
 Next, you should write a conversion script that lets you convert the checkpoint you used to debug *brand_new_bert* in