tests: Fix flaky test for NLLB-MoE (#22880)

* add test update and docs edits

* docs edit suggestion

docs/source/en/model_doc/nllb-moe.mdx

@@ -43,9 +43,10 @@ This model was contributed by [Arthur Zucker](https://huggingface.co/ArtZucker).
 The original code can be found [here](https://github.com/facebookresearch/fairseq).
 
 ## Implementation differences with SwitchTransformers
-The biggest difference is the way the tokens are routed. NLLB-MoE uses a `top-2-gate` which means that blah blah blah blah. 
-In SwitchTransformers, once the masks are computed for each experts, we just index the current hidden_states with the routing mask, and feed the 
-correct tokens to the expert. However here, the implementation varies a lot as the fairseq repository used a different approach.
+The biggest difference is the way the tokens are routed. NLLB-MoE uses a `top-2-gate` which means that for each input, only the top two experts are selected based on the 
+highest predicted probabilities from the gating network, and the remaining experts are ignored. In `SwitchTransformers`, only the top-1 probabilities are computed, 
+which means that tokens have less probability of being forwarded. Moreover, if a token is not routed to any expert, `SwitchTransformers` still adds its unmodified hidden 
+states (kind of like a residual connection) while they are masked in `NLLB`'s top-2 routing mechanism. 
 
 ## Generating with NLLB-MoE
 The avalable checkpoints requires around 350GB of storage. Make sure to use `accelerate` if you do not have enough RAM on your machine.

src/transformers/models/nllb_moe/configuration_nllb_moe.py

@@ -52,7 +52,7 @@ class NllbMoeConfig(PretrainedConfig):
         decoder_ffn_dim (`int`, *optional*, defaults to 4096):
             Dimensionality of the "intermediate" (often named feed-forward) layer in decoder.
         encoder_ffn_dim (`int`, *optional*, defaults to 4096):
-            Dimensionality of the "intermediate" (often named feed-forward) layer in decoder.
+            Dimensionality of the "intermediate" (often named feed-forward) layer in encoder.
         activation_function (`str` or `function`, *optional*, defaults to `"gelu"`):
             The non-linear activation function (function or string) in the encoder and pooler. If string, `"gelu"`,
             `"relu"`, `"silu"` and `"gelu_new"` are supported.

src/transformers/models/nllb_moe/modeling_nllb_moe.py

@@ -460,7 +460,7 @@ class NllbMoeSparseMLP(nn.Module):
                 Attention mask. Can be in the causal form or not.
 
         Returns:
-            hidden_states (`torch.Tensor` of shape `(batch_size, sequence_lenght, hidden_dim)`):
+            hidden_states (`torch.Tensor` of shape `(batch_size, sequence_length, hidden_dim)`):
                 Updated hidden states
             router_logits (`torch.Tensor` of shape `(batch_size, sequence_length, num_experts)`):
                 Needed for computing the loss

tests/models/nllb_moe/test_modeling_nllb_moe.py

@@ -21,7 +21,6 @@ import unittest
 
 from transformers import NllbMoeConfig, is_torch_available, set_seed
 from transformers.testing_utils import (
-    is_flaky,
     require_sentencepiece,
     require_tokenizers,
     require_torch,
@@ -210,7 +209,7 @@ class NllbMoeModelTester:
         self.parent.assertTrue(output_from_past_slice.shape[1] == next_tokens.shape[1])
 
         # test that outputs are equal for slice
-        self.parent.assertTrue(torch.allclose(output_from_past_slice, output_from_no_past_slice, atol=1e-2))
+        self.parent.assertTrue(torch.allclose(output_from_past_slice, output_from_no_past_slice, atol=1e-3))
 
     def check_encoder_decoder_model_standalone(self, config, inputs_dict):
         model = NllbMoeModel(config=config).to(torch_device).eval()
@@ -290,10 +289,10 @@ class NllbMoeModelTest(ModelTesterMixin, GenerationTesterMixin, PipelineTesterMi
                 model2, info = model_class.from_pretrained(tmpdirname, output_loading_info=True)
             self.assertEqual(info["missing_keys"], [])
 
-    @is_flaky()
     def test_decoder_model_past_with_large_inputs(self):
-        config_and_inputs = self.model_tester.prepare_config_and_inputs()
-        self.model_tester.create_and_check_decoder_model_past_large_inputs(*config_and_inputs)
+        config, inputs_dict = self.model_tester.prepare_config_and_inputs()
+        config.decoder_sparse_step = 0
+        self.model_tester.create_and_check_decoder_model_past_large_inputs(config, inputs_dict)
 
     def test_encoder_decoder_model_standalone(self):
         config_and_inputs = self.model_tester.prepare_config_and_inputs_for_common()