[P2] Add skeleton for manual reduce

src/problem2/manual_reduce.py

+# problem2/manual_reduce.py
+from mpi4py import MPI
+import numpy as np
+
+def manual_reduce(sendbuf, recvbuf, op, root, comm):
+    """
+    Implementation of MPI_Reduce using tree-based communication.
+    Toplogy:
+      rank i children: 2*i+1, 2*i+2
+      rank i parent: (i-1)//2
+    """
+    rank = comm.Get_rank()
+    size = comm.Get_size()
+    
+    # Start with local data
+    # We assume sendbuf is a numpy array (for addition)
+    # Copy sendbuf to a local accumulator (or directly to recvbuf if leaf, 
+    # but we need a temp buffer for accumulation)
+    
+    # Logic:
+    # 1. Receive from children (if any).
+    # 2. Add to local accumulator.
+    # 3. If rank != root, Send accumulator to parent.
+    # 4. If rank == root, copy accumulator to recvbuf.
+    
+    pass
+
+def main():
+    pass