Merge mscclpp-lang to mscclpp project (#442)

First step to merge msccl-tools into mscclpp repo. In this step will
move all msccl related code, pass the current tests and do some
necessary refactor.

Add `mscclpp.language` module
Add `_InstructionOptimizer` and `DagOptimizer` class to optimize the dag
Add `DagLower` to lower dag to intermediate representation 
Add documents for mscclpp.language
Remove msccl related code

python/mscclpp/language/rank.py

@@ -0,0 +1,33 @@
+# Copyright (c) Microsoft Corporation.
+# Licensed under the MIT License.
+
+from dataclasses import dataclass, field
+from typing import Dict
+
+
+class BarrierInfo:
+    def __init__(self, tb_list):
+        self.tb_list = tb_list
+
+    def __eq__(self, other):
+        return self.tb_list == other.tb_list
+
+    def __hash__(self):
+        return hash(tuple(self.tb_list))
+
+
+@dataclass
+class Rank:
+    rank_id: int
+    current_max_barrier_id: int = 0
+    current_barriers: Dict[BarrierInfo, int] = field(default_factory=dict)
+
+    def get_barrier_id(self, tb_list):
+        barrier_info = BarrierInfo(tb_list)
+        if barrier_info in self.current_barriers:
+            return self.current_barriers[barrier_info]
+        else:
+            self.current_barriers[barrier_info] = self.current_max_barrier_id
+            barrier_id = self.current_max_barrier_id
+            self.current_max_barrier_id += 1
+            return barrier_id