refine the initial cpu memory flag for mkldnn

[tensor-tang]

resolve #11481

[tensor-tang]

When enabled WITH_MKLDNN:

python test_fit_a_line.py

When set from env:

FLAGS_initial_cpu_memory_in_mb=300 py test_fit_a_line.py

[Superjomn]

Add some comments to tell why 1000 works.

[tensor-tang]

Actually, I am not sure why this exactly 1000 number may works.

I was trying to give more space, here gives 2x of original, for MKLDNN according to the comment from mozga-intel.

And that's why I add @mozga-intel to help to review.

Please @mozga-intel, could you please help check if it's this enough.

BTW, I add this flag to env, so users can change this from env if this default 1000MB is not enough.

[mozga-intel]

@tensor-tang Of course.

[mozga-intel]

@tensor-tang IMO I think that 1000MB of space of memory is too small. The good result is obtained when the memory is greater than 5000 MB

[tensor-tang]

OK, thanks @mozga-intel, I will replace it with 5000 MB.

[luotao1]

LGTM! Thanks @mozga-intel as well.