[Fixed]-Is it possible to show cleaned value on invalid form in django?

In Django form instance can be either bound or unbound. So what you can do is fetch username from request convert it to lowercase and return bounded form with lowercased username.
FYI

def login(request, template_name='login.html',
          redirect_field_name=REDIRECT_FIELD_NAME,
          authentication_form=AuthenticationForm,
          current_app=None, extra_context=None):
    current_site = get_current_site(request)
    if request.method == "POST":
        username = request.POST.get('username').lower()
        pass = request.POST.get('password')
        form = authentication_form(request, data={'username':username, 'password':pass})
        if form.is_valid():
            #Login user and redirect to respective page
        else:
            context = {'form': form}
            return TemplateResponse(request, template_name, context,
                                            current_app=current_app)
    else:
        form = authentication_form(request)
    context = {'form': form}
    return TemplateResponse(request, template_name, context,
                            current_app=current_app)