[tutorial] edited hands-on practices (#1899)

* Add handson to ColossalAI.

* Change names of handsons and edit sequence parallel example.

* Edit wrong folder name

* resolve conflict

* delete readme

examples/tutorial/sequence_parallel/loss_func/bert_loss.py

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+import torch
+import torch.nn as nn
+from colossalai.core import global_context as gpc
+from colossalai.context import ParallelMode
+from colossalai.logging import get_dist_logger
+import torch.nn.functional as F
+import torch.distributed as dist
+from .cross_entropy import vocab_cross_entropy
+
+
+class BertLoss(nn.Module):
+
+    def forward(self,
+                lm_loss,
+                sop_logits,
+                loss_mask,
+                sentence_order):
+        lm_loss_ = lm_loss.float()
+        loss_mask = loss_mask.float()
+        loss_mask_sum = loss_mask.sum()
+        lm_loss = torch.sum(
+            lm_loss_.view(-1) * loss_mask.reshape(-1))
+
+        lm_loss /= loss_mask_sum
+
+        torch.distributed.all_reduce(
+            lm_loss,
+            group=gpc.get_group(ParallelMode.SEQUENCE)
+        )
+
+        if sop_logits is not None:
+            sop_loss = F.cross_entropy(sop_logits.view(-1, 2).float(),
+                                       sentence_order.view(-1),
+                                       ignore_index=-1)
+            sop_loss = sop_loss.float()
+            loss = lm_loss + sop_loss * gpc.get_world_size(ParallelMode.SEQUENCE)
+        else:
+            sop_loss = None
+            loss = lm_loss
+
+        return loss