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/test/configs/mscclpp_lang_test_config.json

@@ -0,0 +1,34 @@
+[
+    {
+        "filename": "allgather_barrier.py",
+        "args": ["8", "8"]
+    },
+    {
+        "filename": "allreduce_allpairs_packet.py",
+        "args": ["8", "8"]
+    },
+    {
+        "filename": "allreduce_allpairs_get.py",
+        "args": ["8", "8"]
+    },
+    {
+        "filename": "allreduce_allpairs.py",
+        "args": ["8", "8"]
+    },
+    {
+        "filename": "allreduce_ring.py",
+        "args": ["8", "8"]
+    },
+    {
+        "filename": "send_recv_packet.py",
+        "args": ["2"]
+    },
+    {
+        "filename": "send_recv_proxy.py",
+        "args": ["2"]
+    },
+    {
+        "filename": "allreduce_nvls.py",
+        "args": ["8", "2"]
+    }
+]