Bump flagtree version for NVIDIA (#5444)

update flagtree to release 0.6.1

Author: tengqm

src/flag_gems/backends.yaml

@@ -51,7 +51,7 @@ backends:
     python: "3.12"
     cmake_backend: CUDA
     triton: triton==3.6.0
-    flagtree: flagtree==0.6.0
+    flagtree: flagtree==0.6.1
     env:
       PATH: /usr/local/cuda/bin:$PATH
       LD_LIBRARY_PATH: /usr/local/cuda/lib64:$LD_LIBRARY_PATH
@@ -64,7 +64,7 @@ backends:
     python: "3.12"
     cmake_backend: CUDA
     triton: triton==3.6.0
-    flagtree: flagtree==0.6.0
+    flagtree: flagtree==0.6.1
     env:
       PATH: /usr/local/cuda/bin:$PATH
       LD_LIBRARY_PATH: /usr/local/cuda/lib64:$LD_LIBRARY_PATH