* add string tensor and case convert kernels

* Add strings empty kernel; Reorganize the structure of case convert kernel

* Add string infermeta

* Update mutable_data of string tensor

* rename kernel name

* add string copy tmp

* Fix strings copy device bug

* add utf8 gpu converter

* add string tensor c++ api

* Remove mutable_data of string tensor

* update string tensor interface

* remove charcases_flag.h

* remove some fluid headers

* Add make_ddim

* __HIPCC__ -> PADDLE_WITH_HIP

* remove fluid headers

* fix cpu compile

* remove std::hash

* Fix cudaMalloc

* Remove strings/impl directory

* Fix infrt/get_phi_kernel_info.py;Add custom_kernels deps

* Add empty kernel test

* Remove some comments

* Modify lower/upper api encoding type: string->bool

* STRING->PSTRING; Add CreateInferLikeMeta

* Add code gen for C++ String API

* remove strings_api_utils.h

* Add ignore file (strings_api.h, strings_api.cc)

* update strings gen script

* change args order of case convert kernels

* Add comments for pstring, StringTensor

* cpstring_internal.h -> cpstring_impl.h

* Update accordding to comments:

1. Remove fluid headers
2. paddle::platform::errors -> phi::errors
3. Use 'place.GetType() == phi::AllocationType::GPU' instead of 'paddle::platform::is_cpu_space()'
4. Use camel code style

* Remove all singletons in strings kernels

* fix rocm compile

* Fix py3 compile

* Fix c++ coverage

* 1. Add pstring proto type
2. Add StringTensor debug info
3. Rename case_convert_kernel to strings_lower_upper
4. Remove serialize derialize strings kernel

* DataLayout::PSTRING -> DataLayout::PSTRING_UNION

* Register pstring data type

* Fix strings api gen

* Fix dense tensor register pstring dtype

* Fix error messages

* remove line

* add pstring unittest

* remove test string api unitest

* remove empty line

* Remove some headers to decrease the size of executable file