[dataset] fix stream is none

[Mddct]

when tarfile open failed, stream will cause "UnboundLocalError: local variable 'stream' referenced before assignment"

[kobenaxie]

建议用

with tarfile.open(fileobj=sample['stream'], mode="r:*") as stream:
    ...

[xingchensong]

The unit test failure is attributed to the issue documented at microsoft/DeepSpeed#4984

[xingchensong]

建议用

with tarfile.open(fileobj=sample['stream'], mode="r:*") as stream:
    ...

后面整体改io的时候再一并修改