Server is not forbidden to create new partition which tag is "_default"

[BossZou]

Describe the bug
Currently each collection has a default partition which tag is "_default". However, user still can create a partition with tag "_default" successfully. This behavior is confusing.

>>> client = Milvus()
>>> client.create_collection({"collection_name": "test_default_partition", "dimension": 128})
>>> status = client.create_partition("test_default_partition", "_default")
Status(code=0, message='OK')

[yhmo]

return error code=SERVER_INVALID_PARTITION_TAG, message="'_default' is built-in partition tag"

[yhmo]

fixed in pr 1770