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_packet.py

@@ -0,0 +1,69 @@
+# 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):
+    """
+    AllReduce with all pairs algorithm using packets format.
+    Steps:
+    1. Each rank sends its nth chunk to the nth rank's scratch space.
+    2. Each rank performs a local reduction on its nth chunk using data from all other ranks' scratch spaces.
+    3. Each rank sends the reduced data to all other ranks' scratch spaces.
+    4. Each rank retrieves the final reduced result from the scratch space.
+    """
+    size = gpus
+    chunksperloop = gpus * gpus
+    collective = AllReduce(size, chunksperloop, True)
+    with MSCCLPPProgram(
+        "allreduce_packets",
+        collective,
+        size,
+        instances,
+        protocol="LL",
+        use_double_scratch_buffer=True,
+    ):
+        # Each rank sends the nth chunk to the nth rank into scratch space
+        for r1 in range(size):
+            for tb in range(size):
+                if tb == r1:
+                    continue
+                remote_rank = tb
+                index = remote_rank * size
+                c = chunk(r1, Buffer.input, index, size)
+                c.put_packet(remote_rank, "scratch", index=r1 * size, sendtb=tb)
+
+        # Each rank performs a local reduction on the nth chunk
+        # Utilize 8 threadblocks for this reduction for better parallelism
+        for r in range(size):
+            for index in range(size):
+                c = chunk(r, Buffer.input, r * size + index)
+                for peer in range(size):
+                    if peer != r:
+                        c.reduce_packet(chunk(r, "scratch", peer * size + index), recvtb=index)
+                for peer in range(size):
+                    if peer != r:
+                        c.put_packet(peer, "scratch", (size * size) + r * size + index, sendtb=index)
+
+        # Each rank get final result from scratch space
+        for r in range(size):
+            for peer in range(size):
+                if peer != r:
+                    c = chunk(r, "scratch", size * size + peer * size, size)
+                    c.copy_packet(r, Buffer.input, peer * size, sendtb=peer)
+
+        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")
+
+args = parser.parse_args()
+
+allreduce_allpairs(args.num_gpus, args.instances)