[misc] Enable offline cache in frontend instead of C++ Side (#6051)

Related issue = #4401 

Enabling offline cache in C++ Side (setting
`CompileConfig::offline_cache` as `true` by default) is dangerous. The
offline cache will always be enabled when running taichi_cpp_tests
because the value of compile config in C++ side is not affected by env
variables, which is not expected and can cause some strange bugs because
it will unexpectedly load cache files that were not expected to
generate.

python/taichi/lang/misc.py

@@ -348,6 +348,7 @@ def init(arch=None,
     current_dir = os.getcwd()
 
     cfg = impl.default_cfg()
+    cfg.offline_cache = True  # Enable offline cache in frontend instead of C++ side
     # Check if installed version meets the requirements.
     if require_version is not None:
         check_require_version(require_version)

taichi/program/compile_config.h

@@ -96,7 +96,7 @@ struct CompileConfig {
   int auto_mesh_local_default_occupacy{4};
 
   // Offline cache options
-  bool offline_cache{true};
+  bool offline_cache{false};
   std::string offline_cache_file_path{get_repo_dir() + "ticache"};
   std::string offline_cache_cleaning_policy{
       "lru"};  // "never"|"version"|"lru"|"fifo"