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/examples/allreduce_allpairs.py

@@ -0,0 +1,65 @@
+# Copyright (c) Microsoft Corporation.
+# Licensed under the MIT License.
+
+import argparse
+from mscclpp.language import *
+from mscclpp.language.collectives import AllReduce
+from mscclpp.language.buffer import Buffer
+
+
+def allreduce_allpairs(gpus, instances, protocol):
+    """
+    Demonstrate allreduce with all pairs algorithm using put semantics.
+    Steps:
+    1. Sync all ranks to ensure the data is ready.
+    2. Each rank reads chunks from all peers and reduces the data.
+    3. Put the reduced data to all peers.
+    4. Sync all ranks to ensure the data is received.
+    """
+    size = gpus
+    chunksperloop = gpus * gpus
+    collective = AllReduce(size, chunksperloop, True)
+    with MSCCLPPProgram("allreduce_pairs", collective, size, instances, protocol=protocol):
+        for rank in range(size):
+            for tb in range(size):
+                index = rank * size
+                c = chunk(rank, Buffer.input, index + tb)
+                # step1 make sure the data is ready
+                for nghr in range(size):
+                    peer_index = nghr * size
+                    if rank != nghr:
+                        # signal peer the buffer is ready
+                        c_peer = chunk(rank, Buffer.input, peer_index + tb)
+                        c_peer.signal(nghr, Buffer.input, peer_index + tb, sendtb=tb)
+                for nghr in range(size):
+                    if rank != nghr:
+                        c.wait(nghr, Buffer.input, index + tb, recvtb=tb)
+                # step2 reduce the chunks and send to peers
+                for nghr in range(size):
+                    if rank != nghr:
+                        c.reduce(chunk(nghr, Buffer.input, index + tb), recvtb=tb)
+                for nghr in range(size):
+                    if rank != nghr:
+                        c.put(nghr, Buffer.input, index + tb, sendtb=tb)
+                # step3 signal the peers buffer is ready
+                for nghr in range(size):
+                    if rank != nghr:
+                        c.signal(nghr, Buffer.input, index + tb, sendtb=tb)
+                for nghr in range(size):
+                    if rank != nghr:
+                        peer_index = nghr * size
+                        c_peer = chunk(rank, Buffer.input, peer_index + tb)
+                        c_peer.wait(nghr, Buffer.input, peer_index + tb, recvtb=tb)
+
+        Json()
+        Check()
+
+
+parser = argparse.ArgumentParser()
+parser.add_argument("num_gpus", type=int, help="number of gpus")
+parser.add_argument("instances", type=int, help="number of instances")
+parser.add_argument("--protocol", type=str, default="Simple", choices=["Simple"], help="Protocol")
+
+args = parser.parse_args()
+
+allreduce_allpairs(args.num_gpus, args.instances, args.protocol)