Update float16 tutorial

[Ishitori]

Description

Some word smiting on float16 tutorial. Fixed the code by adding imports. Added extra examples how to download the data

[roywei]

@Ishitori Thanks for the contribution! I noticed there is additional requirement from import tarfile , please add it to docker install list. Also I would like to request run nightly test on tutorial once to make sure this change is passing. It's not tested in PR validation.
Nightly tests has been failing for a long time, and often due to forget to add dependencies in docker or warnings in the code execution (considered as failure in nightly test). Refer to fix in #15039

[abhinavs95]

@mxnet-label-bot add [Doc]

[Ishitori]

@roywei, I completely agree that we should make sure that nightly tests are working. However, this particular tutorial is a part of FAQ section, which is currently not covered by automatic tests. The tests covers only items under tutorials/ folder. I assume the reason for that is that FAQ items may have some incomplete parts of the code, as for example, this one.

I have addressed comments by @vishaalkapoor made in another copy of this PR (closed now).

[thomelane]

@Ishitori can you link to the other PR? Would be good to see comments here.

[Ishitori]

@thomelane mli/new-docs#114

[thomelane]

LGTM