[BUG] fix SymbolBlock constructor from user-defined graph

[khaotik]

Description

Fixes a bug where SymbolBlock does not copy user-given parameter attributes such as lr_mult. Simple reproducer:

#!/usr/bin/env python

import numpy as np
import mxnet as mx
import mxnet.symbol as mxs

DTYPE = np.float32
LR_MULT = 0.555
WD_MULT = 0.444

s_x = mxs.var('x', shape=(1,256,), dtype=DTYPE)
s_w = mxs.var('W', shape=(256,192), lr_mult=LR_MULT, wd_mult=WD_MULT, dtype=DTYPE)
s_b = mxs.var('b', shape=(1,192,), dtype=DTYPE, init=mx.init.Zero())
s_y = mxs.linalg.gemm(s_x, s_w, s_b)

fn = mx.gluon.SymbolBlock([s_y], [s_x])
fn.initialize()
fn.forward(mx.nd.random_uniform(-1., 1., shape=(1,256), dtype=DTYPE))

param_di = fn.collect_params()

assert param_di['W'].lr_mult == LR_MULT
assert param_di['W'].wd_mult == WD_MULT
assert not param_di['b'].data().asnumpy().any()

Checklist

Essentials

• [x] PR's title starts with a category (e.g. [BUGFIX], [MODEL], [TUTORIAL], [FEATURE], [DOC], etc)
• [ ] Changes are complete (i.e. I finished coding on this PR)
• [x] All changes have test coverage
• [x] Code is well-documented

Changes

• [x] SymbolBlock now copies user-given wd_mult lr_mult init attributes from symbol.
• [ ] stype attribute

Comments

• stype attribute is still not copied. There's no MXSymbolInferStorageType C API. I think a complete fix would require some C API mods.

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