* [Eager]Polish enable/disable_legacy_dygraph logic

* fix test_var_base print_tensor

* fix bug caused by arange

* Updated bf16 cast case

* BF16 astype to float32
Co-authored-by: NAurelius84 <zhangliujie@baidu.com>
Co-authored-by: Npangyoki <pangyoki@126.com>
Co-authored-by: Nzyfncg <zhangyunfei07@baidu.com>