[Trivial] Fix NDK warning -Wbraced-scalar-init

[Trivial] Fix NDK warning -Wbraced-scalar-init

This patch fixes trivial NDK warning below

```
././../nntrainer/layers/lstm.cpp:315:25: warning: braces around scalar initializer [-Wbraced-scalar-init]
  Tensor dh_nx = Tensor({derivative_.width()});
                        ^~~~~~~~~~~~~~~~~~~~~
././../nntrainer/layers/lstm.cpp:316:25: warning: braces around scalar initializer [-Wbraced-scalar-init]
  Tensor dc_nx = Tensor({derivative_.width()});
                        ^~~~~~~~~~~~~~~~~~~~~
././../nntrainer/layers/lstm.cpp:352:26: warning: braces around scalar initializer [-Wbraced-scalar-init]
        hs_prev = Tensor({hs_t.width()});
                         ^~~~~~~~~~~~~~
././../nntrainer/layers/lstm.cpp:354:26: warning: braces around scalar initializer [-Wbraced-scalar-init]
        cs_prev = Tensor({cs_t.width()});
```

**Self evaluation:**
1. Build test: [X]Passed [ ]Failed [ ]Skipped
2. Run test: [X]Passed [ ]Failed [ ]Skipped

Signed-off-by: Jihoon Lee <jhoon.it.lee@samsung.com>