Don't hash the pointer to the real function when generating the offline cache key

[lin-hitonami]

Describe the bug
When we get the offline cache key, we use the value of Function * as the key of the real function. However, this changes when we reset Taichi. We should use the hash key of the IR of the real function instead.

taichi/taichi/analysis/gen_offline_cache_key.cpp

Line 243 in 9cbc59a

emit(expr->func);

[PGZXB]

FYI, @lin-hitonami

taichi/taichi/analysis/gen_offline_cache_key.cpp

Lines 509 to 519 in 9cbc59a

	void emit(Function *func) {
	TI_ASSERT(func);
	auto iter = real_funcs_.find(func);
	if (iter != real_funcs_.end()) {
	emit(iter->second);
	} else {
	auto [iter, ok] = real_funcs_.insert({func, real_funcs_.size()});
	TI_ASSERT(ok);
	emit(iter->second);
	}
	}

taichi/taichi/analysis/gen_offline_cache_key.cpp

Lines 433 to 440 in 9cbc59a

	// Serialize dependent real-functions
	emit(real_funcs_.size());
	for (auto &[func, id] : real_funcs_) {
	if (auto &ast_str = func->try_get_ast_serialization_data();
	ast_str.has_value()) {
	emit_bytes(ast_str->c_str(), ast_str->size());
	}
	}

[PGZXB]

However, the algorithm that generates the AST key for the real function is indeed wrong. See #6877. I will redesign it.

[lin-hitonami]

Okay, I missed that part.