Change default datetime index resolution to ns to match pandas

[vyasr]

The default datetime resolution in cudf has been ms since we introduced datetimes because we did not initially support different resolutions. Now we can change this to match pandas.

[codecov]

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@@               Coverage Diff               @@
##             branch-21.08    #8611   +/-   ##
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  Coverage                ?   83.00%           
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  Files                   ?      109           
  Lines                   ?    18239           
  Branches                ?        0           
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  Hits                    ?    15140           
  Misses                  ?     3099           
  Partials                ?        0           

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[vyasr]

@gpucibot merge