[tutorial] added missing dummy dataloader (#1944)

examples/tutorial/.gitignore

-data/
+./data/

examples/tutorial/sequence_parallel/data/dummy_dataloader.py

+import torch
+
+
+class DummyDataloader():
+
+    def __init__(self, batch_size, vocab_size, seq_length):
+        self.batch_size = batch_size
+        self.vocab_size = vocab_size
+        self.seq_length = seq_length
+        self.step = 0
+
+    def generate(self):
+        tokens = torch.randint(low=0, high=self.vocab_size, size=(
+            self.batch_size,
+            self.seq_length,
+        ))
+        types = torch.randint(low=0, high=3, size=(
+            self.batch_size,
+            self.seq_length,
+        ))
+        sentence_order = torch.randint(low=0, high=2, size=(self.batch_size,))
+        loss_mask = torch.randint(low=0, high=2, size=(
+            self.batch_size,
+            self.seq_length,
+        ))
+        lm_labels = torch.randint(low=0, high=self.vocab_size, size=(self.batch_size, self.seq_length))
+        padding_mask = torch.randint(low=0, high=2, size=(self.batch_size, self.seq_length))
+        return dict(text=tokens,
+                    types=types,
+                    is_random=sentence_order,
+                    loss_mask=loss_mask,
+                    labels=lm_labels,
+                    padding_mask=padding_mask)
+
+    def __iter__(self):
+        return self
+
+    def __next__(self):
+        return self.generate()
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