* Add core.eager.StringTensor __init__ which pyarray args can be passed

* Add the numpy method of core.eager.StringTensor

* revert tensor.to_string modification

* Add ToPyObject for core.eager.StringTensor

* Add debug string for core.eager.StringTensor

* Remove place args of core.eager.StringTensor temporarily

* Fix check string_tensor error

* remove dtype of core.eager.StringTensor

* add core.eager.StringTensor unittest

* remove pstring from VarDesc

* Add InitStringTensorWithStringTensor

* Remove to_string modification

* Remove zero_copy arg from StringTensor creator