synchronize before print bf16 cast

[wanghuancoder]

PR Category

Execute Infrastructure

PR Types

Bug fixes

Description

bf16由于类型支持问题，在print的时候需要转成fp32，以使用numpy的功能。
但Tensor.numpy是有Sync的。cast没有。在多流情况下，主流CPU，print分流Tensor时，如果cast没有Sync会在Tensor没有运算完就cast。会导致精度异常。

Pcard-67164

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[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (develop@8ad991d). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
python/paddle/tensor/to_string.py	0.00%	4 Missing ⚠️

❌ Your patch status has failed because the patch coverage (0.00%) is below the target coverage (90.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@            Coverage Diff             @@
##             develop   #74278   +/-   ##
==========================================
  Coverage           ?    0.00%           
==========================================
  Files              ?        1           
  Lines              ?        4           
  Branches           ?        0           
==========================================
  Hits               ?        0           
  Misses             ?        4           
  Partials           ?        0           

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[DanielSun11]

LGTM