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First you need to understand what the following code does. It creates a column of width 9/12 the width of the page.
<div class="col-md-9">
You might wan to do the following.
<div class="col-md-9">
<div class="form-group">
{{ field|addcss:"form-control" }}
<button type="submit" class="btn btn-primary btn-sm">Filter</button>
</div>
</div>
Give explicit width to the button. Check online to give width inline
Source:stackexchange.com