mxnet cannot save parameter with name starting with slash

Description

A minor issue: mx.numpy_extension.savez does not accept array-dict with a key starting with slash. This started to occur as soon as I upgraded to master fork. It is also inconsistent with numpy behavior.

This function is used by gluon.Block.save_parameters methods.

Error Message

Traceback (most recent call last):
  File "test.py", line 12, in <module>
    mx.numpy_extension.savez('/tmp/m_mx', **{param_name:m})
  File "/media/LNXDATA/WKSP/dev/incubator-mxnet/python/mxnet/numpy_extension/utils.py", line 122, in savez
    check_call(_LIB.MXNDArraySave(c_str(file), mx_uint(len(handles)), handles, keys))
  File "/media/LNXDATA/WKSP/dev/incubator-mxnet/python/mxnet/base.py", line 246, in check_call
    raise get_last_ffi_error()
mxnet.base.MXNetError: Traceback (most recent call last):
  File "/home/khaotik/WKSP/dev/incubator-mxnet/src/serialization/cnpy.cc", line 333
MXNetError: Check failed: mz_zip_writer_add_read_buf_callback(archive, blob_name_npy.data(), npy_header_blob_read_callback, static_cast<void*>(&callback_data), size_to_add, nullptr, nullptr, 0, MZ_NO_COMPRESSION, nullptr, 0, nullptr, 0): invalid filename

To Reproduce

#!/usr/bin/env python
import numpy as np
import mxnet as mx

m_npy = np.random.rand(256,256).astype('float32')
m = mx.ndarray.from_numpy(m_npy)
param_name = '/op/m' # starts with a slash

np.savez('/tmp/m_np', **{param_name:m_npy}) # WORKS
mx.numpy_extension.savez('/tmp/m_mx', **{param_name:m}) # BROKEN

What have you tried to solve it?

In file python/mxnet/numpy_extension/utils.py, substitute NDArraySave with NDArrayLegacySave provides a temp fix. Also, the issue can be avoided by not using slash as parameter name prefix.

Environment

ubuntu 20.04 anaconda py 3.8.3 mxnet master branch commit cb5bd4ea8b6fc9b568c13747bdb006ac047b72b5

npz format is based on the zip file standard. In zip files, filenames beginning with a / are not allowed.

4.4.17 file name: (Variable)

   4.4.17.1 The name of the file, with optional relative path.
   The path stored MUST NOT contain a drive or
   device letter, or a leading slash.  All slashes
   MUST be forward slashes '/' as opposed to
   backwards slashes '\' for compatibility with Amiga
   and UNIX file systems etc.  If input came from standard
   input, there is no file name field.

https://pkware.cachefly.net/webdocs/casestudies/APPNOTE.TXT

For reference, we're using below zip implementation which, unlike Python / Numpy, enforces the filename requirements.

https://github.com/richgel999/miniz/blob/2281a42c77a08188a57609c8950b447ec62d6fb8/miniz_zip.c#L3109-L3113

Unfortunately this looks like a bug in numpy. We can consider following numpy here in writing non-standard zip files, but that's not supported by our current zip implementation.

apache / mxnet