[xla-auto-sharding] Move implementation of CostGraph methods to .cc f…

…ile.

Notes:
- Create auto_sharding_cost_graph.cc and move CostGraph implementations there.
- Make ToString() a const method.
- Make most pass-by-value arguments const.
- Add TODO to make class members private.
PiperOrigin-RevId: 615451743

third_party/xla/xla/hlo/experimental/auto_sharding/BUILD

@@ -143,6 +143,7 @@ cc_library(
 
 cc_library(
     name = "auto_sharding_cost_graph",
+    srcs = ["auto_sharding_cost_graph.cc"],
     hdrs = ["auto_sharding_cost_graph.h"],
     compatible_with = get_compatible_with_libtpu_portable(),
     deps = [